OK, I've been caulking things for over 20 yrs (glass business) it takes a LOT of practice to be able to run a nice bead.
I've seen a few posts that tell you to practice running beads, but that is a waste of caulk which is going to cost you even more money.
If you have at least a halfway decent caulking gun, I would recommend using tape as others have pointed out (you can use cheap masking tape btw instead of that expensive blue painters tape) and getting one of those finishing tools from Home Depot like this one