Use Caulk that is a blend of acrylic and silicone. Choose a color that is close to the color you want to paint. The tools required are a caulk gun, a putty knife, a razor knife, a tile-grout sponge and a bucket. There are options in caulk guns ranging from cheap to fairly expensive. The more caulk comes out of the gun at once, the less control there is. Apply caulk after primer is applied because it is difficult to remove caulk from wood grain. Try to use small amounts of caulk at a time.

How To Caulk Baseboards