My Guy, just tell her u love her and you want to spend the rest of your life with her. If u want to put some romance into it you could take her out say on a saturday, whenever you know you'll have a lot of time to spare, spend time with her doing the things you love to do toghether, then end the day with a nice dinner and then pop the question.
This is just one way, but i'm sure whichever way you choose to do it she'll be thrilled. Good luck Man
