access another computer remotely

I want to be able to access my mothers computer from my computer to help her out when she has problems. I live in CA and she is in CO. it is just too hard to try to explain anything to her on the phone. How would I go about setting that up. I think it's set up (I did set up the computer a few years ago) but she recently got Internet. it has been a while since I did anything like that so just need a refresh. I have an updated MacBook and she has an older Mac laptop. Thanks