HI Scott and welcome
As a
tiler I use mainly my finger and soapy water sprayed on using a spray bottle. If you havent done this much before as there is a slight skill to it and you want the best finish then I would suggest masking taping the area to silicione. along both surfaces, this will give you a nice straight edge to the silicone.
Line the silicone, then either using a soapy finger or spit run your finger along the silicone using even pressure. Remove the masking tape as soon as you have done this though and before it skins.
You might want to do one line at a time.
Alternatively you can buy a silicone profiling tool, Wickes sell a good one but if your new still use the masking tape method. I prefer my finger as you can feel the correct pressure to apply etc.
Ive found Screw-fixes no nonsense brand easy to use and cheap enough.