The first thing I do is approach it from the perspective of "here's the awesome stuff I/we did recently" and show some recent adventure you had with great views. It can help to tell them how much you yourself benefit from a relationship with the outdoors.
Then it really depends on your friends. If they're into fitness, approach it from that angle, if they're into photography, that works too; If they like the idea of travelling, tell them about the wealth of experiences they're missing out that are nearby, etc.
Then when you've found some common ground and agreed to take them on your favorite hike/bike ride/etc., you need to focus on putting extra effort, taking the brunt of preparing the logistics, etc. so that for their first few experiences, they get a bit of luxury treatment, not having to worry too much while relying on your expertise.
And on that first experience, make sure it's one of your favorites. For me it's mountain lakes and vistas, in your area it might be something else.
I don't necessarily agree with the "start easy" thing, I think it really depends on the person, but mostly you want to give them an experience they won't forget, get them "hooked", in a way. On the first hike/trip whatever, take lots of pictures. It helps people associate with something if they photos of themselves doing it. Keep your friends engaged and be attentive to their comfort level/etc.
Sometimes, the challenge is in and of itself a way to get them hooked, depending on the personality.