Do Talk about Marriage
Before you propose, talk about the possibility of getting married. Make sure it's something that you both want.

Do make it a Surprise
Even though you have discussed marriage, the proposal itself should still be a surprise. Find a moment and a way that is special and she should not be expecting it. Remember it should be a surprise.

Do Pick a Personal Spot
Think about your favorite romantic places -- choose a romantic spot to pop the question. Just make sure that it is a special spot and this is your personal moment.

Do Be Creative
Incorporate your personality, favorite things, foods etc. into the proposal. Make it memorable and something to cherish the rest of your lives.