Teamviewer is the easiest and most reliable way. The way it works is this:
You each install Teamviewer (or run the application without installing, it gives you this option when you double-click the executable).
After accepting the license agreement Teamviewer's main will then show and you'll be given an ID and password on the left side (your grandma will need to tell you this). On the right hand side you will use the "Partner ID" box and enter in your grandma's ID, then the password when it prompts you. Leave it set to "Remote control".
Once you've done this, your grandma's desktop will then pop up and you'll have full control over her system as if you were sitting in front of it! Be sure to remind her not to touch the mouse or keyboard while you're working, as she'll still be able to control them but that will of course override your control.