click on Add and you can enter these values
in the second CNAME section ( which is erased in the pic ) is particular to your blog and your Google Account, and is therefore different for each person, you can find it by following the step 2.
1. First go to settings, then to basic
2. Now click + Setup a 3rd party URL for your blog
3. Now enter your domain that you bought ( here in the example i bought a domian from godaddy )
4. Press save and you are good to go.