The quick answer is get yourself to an airbag and get around coaches who know how tricks work mechanically, personally I'd suggest WV BBTS, they have camps in the spring, fall, and summer and their coaches know what they're talking about.
The longer answer is: 1) make sure you have a sound Cork 7 (Assuming spinning L, if spinning R the same principles apply just mirror the directions... obviously lol) with proper lean on takeoff, what that means is that you should make sure that when you take off on a cork 7 your body is leaning left, for example if you draw a straight line down your body (looking down hill) it should be somewhere between \ and I. if your body lean looks like this, / , you'll probably have an issue staying at 7 (the trick will have a tendency to want to go to 9) and when it does stay at 7 the way it comes around puts lots of stress on your knees, specifically ACL. Making sure you have a good lean is very important when trying to add a flip because it makes sure you come through into the second flip squarely and keeps you doing the trick you want, not accidentally a 12 or something.
2) Carving isn't an issue however make sure you're not relying on the carve for power. you generate the most powerful and stable rotation by staying strong and setting through your shoulders, core, legs and feet (in an oversimplified way: legs and feet work to generate mostly flip, shoulders and core mostly spin, however all need to work controlled and together). Ideally you should be pressuring on your left edges (not specifically carving but pressuring) this will help the previously mentioned lean thing and help power.
3) generally flippy is easier to learn and then yo can work it more upright if you want but it mostly depends on what you're comfortable with and what you're comfortable flipping over in the second flip (ex: cork 3, backflip, and variation in between)
Keep in mind all of this is a serious oversimplification and everything here glosses over a great deal of complexity, also remember its generally guidelines but everyone works differently. once again I'd suggest get yourself to an airbag and/or find coaches, its a lot easier to help in person and not through an NS forum. hope this helps! best of luck!
