Good luck! I am quite the expert having self-colored for almost 20 years. Happy to answer your questions.
Things to remember: Clothes should be easily removable when you hop in the shower and not definitely not go over your head, or they will most certainly stain. Alternatively, you can just lean over the sink and wash your hair only, but I still manage to get stains then and not just on my clothes but all over the bathroom.
And no, you don't get stains in the tub - unless your tub is very old and the enamel (gloss?) has worn off. I've had this happen with very vibrant colors (think dark brown, black, red) in rental units in old and neglected student housing.
Another thing to pay attention to is the back of your head. Do you have long hair? It can be a bit difficult to coat your hair evenly with dye in the back of your head with long hair. You won't get an even look and your hair won't take to the dye if you don't completely drench it.
There is a method, and it would be helpful to have someone to help.
There are also other things to consider: Are you coloring virgin hair or is your hair already colored? Are you covering grays? etc. so read up on those if you haven't already done so to get the best results.