[Arthropraxis]

Laticrete Latasil's 100% silicone caulk, comes in different colors can be bought off amazon. Use painters tape to create a border on either side of the caulk bead, run your finger down the bead to remove the excess, take one side of the tape off, run your finger down again to smooth the edge, remove the other side of the tape and run your finger to smooth that side. Step back and admire your straight caulk lines.