How do you tell your parents you're getting engaged?

I am a sopre in college and my boyfriend is about to join the coast guard in about 3 months. We've been together for about two years and I couldn't be happier. Before he leaves for his new station, we are planning on getting engaged... so that's pretty soon. I already got a promise ring for Christmas so I'm sure my parents know it's coming. The problem lies in that in the begining of our relationship, we hit some rocky times and he had a marajuana problem which is GONE now and he is the same man I knew before that all happened. And we were kinda on again off again for a while.. I know his parents will support our decision as they have already suggested I move with him to his new station.. But we have talked about it and I am going to stay here for a year and then we will get married next summer to give me time to get my cles out of the way, him set up at his new station, and us time to save money. I just don't know how to broach this with my parents...