For me, it is better to ask any company like this one for helping me. Once I tried to replace power sockets in my house, and it finished badly. I will not tell you what happened but believe me, that was an unpleasant situation. Since then, I never do anything connected with house renovation by myself!
Use plumber putty instead of the tub gasket. Roll it like a snake anput it in place.
Clean up up the excess with rag!