No extra underlayment needed. Level the floor and add a vapor barrier then install.

If you apply too much caulk, it doesn't look good. If you don't apply enough caulk, you aren't sealing. The painters tape makes sure the excess caulk gets removed.

Tape just a hair below the top of the baseboard. You should be able to fit a coin standing up, but that's about it. You can also put a bit of tape on the wall just above the baseboard. Your caulk tube should have a relatively small hole to better control the flow of caulk.

Some people do top only, some do top and bottom.

Take some painters tape and lay it down before you caulk. Then caulk. Then, take the tape off before the caulk dries.