OK so this is pretty simple, but many people make caulking more complicated than it should be. You can buy all kinds of gadgets at the store that help you get clean caulk lines. People can overthink caulking, but it doesn’t have to be complex. The solution is actually incredibly easy. Just use painters’ tape and caulk that comes in a squeezable tube. You don’t need a fancy tool to go over the caulk and you don’t need a big fancy caulk gun (unless you have a huge project).
I recently removed the carpet from my staircase and refinished them with paint and wallpaper. I needed to caulk the bottom of each riser. It was so simple to do with painters’ tape and caulk that came in a tube. I just placed the tape near the edge across the tread and then squeezed the caulk out across the seam. I took my finger along the length to spread the caulk evenly into the space. I then immediately pulled up the tape away and had a super clean line! This method can be used on a variety of projects including caulking around your bathtub. In that case just tape both sides with the caulk in the middle.
Happy Caulking!