I found a wonderful post on the Convict Conditioning facebook group page from James Hirshfield. It's re-posted here with permission unedited. His post resonated with me because I'e always been considered "skinny" even into my 30's until I took measures to add weight and muscle. The interesting thing is the eating part, which has been confirmed by many people including Tim Ferris (Four Hour Body, etc.). It gives you a appreciation for how hard it is for many people to LOSE weight, by experiencing your own difficulty in GAINING weight. Anyway, here is James's post. It's one of the best "How to Gain Weight for Skinny Guys" articles Ive read a long time.

No Bullshit Plan - Skinny to Muscular.
This is how I personally went from 6’3” 149lbs to 170 in 12 weeks.
I’m not gonna start this off by saying its easy, in fact its quite the opposite. If you’re reading this, you and I are few are far between.
“You’re so lucky, you can eat whatever you want and you’re still skinny”
“Eugh, I hate you, you’re so skinny”
“People like you don’t understand, it’s so hard to lose weight”
“Pencil legs”
“Treasure chest (sunken)”
“I bet you blow away when its windy”
If you’ve heard any of these, you’re in the boat I was in. These people are so wrapped up in their own world, they don’t realize the possible traumatic chaos it causes in your own head. I would look in the mirror and just see bone. I would see someone equally skinny, and think, “i hate that”
You’re an easy target, and you’re probably your own worst enemy. Well i’m gonna tell you something. If you REALLY want to change, and I mean REALLY want to change, you can do it, because I did. I read all the articles, all the shit about skinny guys and thought it just ISN’T possible. Well, IT IS. and TODAY you are gonna start.
I’m not gonna give you all the science. There is a good chance, you already know how it works.
I'm also not gonna going to tell you to count all your calories, or to count your protein, this is for the real hard gainer, or someone who wants to bulk up FAST, and then get to the details and leaning later.
For now, your goal is to get big. Period.
RULE 1. EAT.
Simple. Eat. and I mean eat like you haven’t seen food in your life before. You can research on youtube, specific “diets” and when I say diet, I’m talking about lifestyle food choices. I tell people all the time I’m on a diet. Just because it doesn't fit into their little world that I diet must mean to starve yourself, take power of that word, you're on a diet to bulk up.
I’m going to tell you pure and simple what I did, and this is extreme, but this is for those who actually WANT results, and are sick of the bullshit name calling, sick of being the skinny guy pushed around. This is for you skinny guys, like me, who set themselves a goal, and will give the middle finger to anyone trying to stop it. Yes, I want to get in your head, and you will need to learn how to do that too. Whether you use anger, fear, love, music, girls to motivate you, you need to find someone strong to push you when the going gets tough.
How to eat? 6-8 meals a day. Don’t be ANAL. don’t sit there counting your bullshit out, save that for later. I want you to eat, and eat FAST and eat often. Reverse the techniques the weight loss people use. There is about a 20 minute delay between when you’re full and when that message gets to your brain, and I want you to bypass that 20 minutes to shove as much damn food in your throat as you can. Now remember, I was 6.3… I mean pretty tall, and not even 150, so I was harsh on myself, there fore a big fuck you to the world and did the following. I would keep eating, at each meal, until my gag reflex kicked in twice. At that point, I knew I was full. Now i know financially that is hard, probably meaning giant meals, so think smart and get high calorie foods, pastas, rice, potatoes, beans, bread, shit loads of lentils, more greens than you can imagine, I don’t care what it is, that food needs to be inside you. You want to be an machine, learn to eat like one.
How do I eat 6-8 meals? I toyed around with some supplements, and some worked. I enjoyed the No2 stuff, and creatine, but I chose that I wanted my results to be from ME. I used my own motivation to pump me up, i want that look in your eye like you’re gonna rip someones head off. That girl that broke up with you and now sleeping with some other dude…? your sister getting treated like shit from her boyfriend….? what you gonna do. You need to harness whatever the fuck it is, to pump you. So no, I didn’t use those supplements for long, though I’m sure they work.
Now there is ONE product that I will recommend. N-Large 2. At the time, this was the BEST high MASS shake out there. With the best sugar to protein ratio, and the best flavor in my opinion, and available in a 10lb tub you can replace every 3 weeks. Between breakfast lunch and dinner, chuck one of those shakes down, and on work out days, again for dessert. After 2 weeks, your stomach will be growling every 3 hours as you’ve conditioned yourself to eat.
Breakfast (boiled eggs, oatmeal, honey, pancakes, bananas, cereal, fruit)
Shake (with or without milk, I don’t care)
Lunch (rice, potatoes sweet/regular, pasta, broccoli, carrots, peas, lentils, asparagus, beef, chicken, fish, more rice, more legumes, bread, pizza
Shake
Dinner (as above)
Shake
Late Night Snack/desert (sandwich, peanut butter, jelly, buckwheat, fruit, leftovers)
Shake
Now above are just examples, you can make your dinners as fancy as you want, or as simple as you want, for me, it wasn’t about how pretty my plate looked, the question was “is there enough on here”
Again, I was pushing to extremities, but I wouldn’t leave until gag reflex twice. Now I don’t wanna read comments about eating disorder and the like, the ONLY reason I ate until that point was because it was only then, that I knew my body was physically full.
So, you got this? Rule 1, eat. A lot, Fast, Frequent. If you don’t get enough greens in in one meal, add some more for the next, woke up late, add an extra meal later, just DO WHAT YOU NEED TO DO FOR FUEL, because you’re going to need it.
RULE 2: LIFT
Now, my research has lead me down many different paths and research papers, such as, calisthenics or free weights, Grease the Groove or Body by Science. They are all brilliant, and they all have one thing in common. You gotta get off your skinny ass and learn to move heavy shit around.
The next section needs to be smart. And please remember, this is my educated opinion. I want you to push to “safe-fail”
What does that mean, that means if you’re doing a bench press, and you only have the bar, and you do 5 reps when you could have done 20, you need to go home. Stop wasting your time. I want it to be heavy enough that you have to really push yourself, you should be pushing your breath out with the last rep, but without compromising form, and with enough in the tank if you really needed to do one more.
I’ll tell you why, and from experience. If you are pushing SO hard, that you’re face is bright red, and your lugging the weight around from ares of the body that shouldn’t be moving like that, you ARE going to get INJURED, and when that happens, say good bye to your weight gain. You’re goal is to GET BIG, not to get in hospital.
Now I am a BIG proponent of Calisthenics, so I believe you can ABSOLUTELY do this, and currently AM doing this, as long as you are at the progression where is is DIFFICULT and you’re doing less than 7 reps. When you get to 7, up the progression, or the weights on the bar, or slow down the pistol squat.
Focus on the BIG MOVEMENTS. Here was my break down, PRECISELY what I did.
5 for the big group, 2 for the small
Monday: Chest + Triceps (primemover and synergist muscles)
Bench Press 3 sets 5-7 reps - 3 minute rest
Incline Bench Press
Decline Bench Press
Flys
Dips
-Tricep Skullcrushers (not the safest name but applicable, be sensible here)
-Tricep Pulldowns
Wednesday: Legs
Squats 3 sets 5-7 Reps
Leg Press
Leg Extension
Glute Ham Raises (face down, heels under a bar, then lower your body in the straight line)
Seated hamstring curl
-Calf Raises (15 reps max, increase the weight)
Friday: Back + Bicep
Deadlift 3 sets 5-7 reps(PLEASE BE CAREFUL WITH THIS, START SLOW AND LEARN, otherwise, do it with a dumbbell, like sumo deadlift / kettle bell swing style
Pullups (use the progression necessary)
Bent over Rows
Seated Rows
Shrugs
-Bicep Curl Dumbbell
-Hammer Curl
Notice these are the BIG MAIN exercises and are ordered as such for good reason, do the big exercises first before the small ones.
For any of these exercises, replace them with the progression as necessary, but make sure the progression is hard enough.
EXTRAS: We all know the other important stuff, or I hope we do. Make sure you sleep, now you’re gonna have your life, sometimes more sleep, some times less, but don’t kill yourself with no sleep, and don’t go slipping a vertebrae by going for a 1RM on a deadlift… trust me..
Alcohol. You know what, go have a beer, enjoy your life. you’re gonna be working hard, and a beer here n there isn’t gonna kill you, it’s gonna have its effect on cortisol etc, but you know what, worry about that later. WATER, drink that clear stuff like you’re never seeing it again.
MOTIVATION: I want you to do 2 things, and this is it. Take a photo at day 1. and Weigh yourself at day 1. I don’t want you to step on a scale until 6 weeks, because if you’re like me, the increment will be small and hard to see at a micro level, but TRUST ME, after 6 weeks you’ll realize why it was worth it.
I wanna recap. 1 EAT. This is critical. You can workout as much as you want and study till the cows come home, but if you don’t eat, expect to look like slim lean bean.
2. LIFT. Safe-Fail. Push hard, but not to injury. That will be your BIGGEST set back, Use the correct progression to cause hypertrophy. You should be in and out of the gym in under 75 minutes. Any longer and you’re weight isn’t heavy enough and you’re doing too many reps, your rest is too long because you’re distracted with your iPhone, or you’re just fucking about.
If you follow this guide. I can ASSURE you, without question, you will bulk up, and bulk up fast.
And when 3 months come, and you surprise your buddies at the beach, pool, or where ever and you take your shirt off, someones gonna tell you “holy shit, when the fuck did that happen!?” and when they say that, you know it’s all worth it.
Hope you enjoy, I’ll make edits along the way if I see fit, but keep the moral high, don’t let people put you down, you have your purpose, now time to stop making shit excuses and get started. Sooner you start, the sooner you’ll get to your goal. Peace