What a coincidence, this is exactly what I am doing this week. Take a look here and this should help you:
This link should help:
One thing to make sure is that you take out all the old caulk or the new will not adhere. Also use 100% silicone in the bathtub. When I did mine I taped off the area then I filled the tub with water to make it heavy. I applied the silicone between the two tapes. Came out great. Let the water stand overnight then drained.
The process is very simple now you can buy a cheap plastic tool that can make your chalking look professional. Clean your surface and add your chalking then remove the excess with your plastic scraper.
these should help
I wrote a whole article about replacing the caulk on a tub! https://www.charlestoncrafted.com/refresh-your-tub-by-replacing-the-caulk/
this video should help https://www.youtube.com/watch?v=eEsy7luzUTI
Buy Caulk and wrap a piece of Wax Paper around your index finger. You apply the caulk like toothpaste and Smooth it with the wax paper.