There's really only one thing you have to do to teach someone how to do their genealogy online. Here it is:
Don't touch their computer.
This can be especially hard for young people who are trying to teach older people how to use family history websites. It is tedious to watch someone else type and figure out where to click. It would be so much easier to just do it for them. But if you have control of the computer the person you are trying to teach will not know what you did when they go home. They will be totally lost. They may give up because of it. Don't touch their computer!
Of course, that's not the only thing you'll want to take into account when you're teaching someone about family history. This Church-produced video shows a great example of a young woman teaching family history.