How do I replace a battery in a watch?

I have a couple of watches that need a new battery. Neither are expensive watches and none are waterproof/resistant. I just like them and do not want to toss them in a landfill.

Can I learn to do this myself and if so where do I get the tool to open the back?

Can anyone here tell me how it is done and how not to mess it up or at least point me in the right direction to figure it out?

(Please do not tell me to go to a jeweler as I am flat broke and the watches have a value of less than the jeweler would charge to do it. Thanks!)