If you can do underflips, you can easily learn cork 3's and 5's. They feel essentially the same, just less flippy than an underflip if that makes sense - carve more off the lip and set the trick in a round horizontal motion rather than dropping into lincoln while spinning.
Cork 7: Practice on a tramp/diving board or anything you've got. Set a decent pop first (don't overdo it, barely popping should be fine for your first attempt) then set the spin. If you spin left, focus on bringing your right hand in front of you, then aiming it towards just behind your left ski heelpiece. Stay relaxed and confident in the air that it will come around - do not freak out, this will only get you hurt and cork 7's usually come around if you can stay relaxed. The first 360 will be blurred, but at 360 you should see the noses/tips of your skis parallel to the ground in front of you. At 360 give a very very light dip to your left shoulder and you should be seeing the knuckle just after this at 540. Bring the spin around to 720, and stomp it. Don't concern yourself with trying to grab your first few times - there's just no need to add extra movement. Upon landing, you'll likely learn that putting forward pressure on your boots is important when you set your pop (you will likely land backseat the first few tries).
Going forward, the easiest grab for me to learn at first was critical/truck/nose grab. Right at 360 when you see the window between your skis, grab em. For some people, leading blunt/tail is the easiest way to set a cork 7. When you set off the lip, a lot of people find it easiest to look for that left tail and grab it quickly - all personal preference, with time you'll learn how to do any grab with cork 7's. Good luck.