Find a little jump and pencil the shit out of them. Sure it doesn't look cool, but it will help you get the rotation down without getting hurt. Work on getting a solid pop, not taking off to early, and keeping a forward stance off of the jump. Even though you can takeoff much more backseat and land a 5 than you can with a 3 it's important to keep your weight forward off the jump. This will especially help when you start to learn 7's. When you pencil you don't have to worry about getting your knees up or grabbing, you're just working on the basics of the trick. Once you've got consistent 5's, work on alternating between 3's and 5's so you can easily and effectively control your rotation in the air.
Now you have the pencil rotations down you can find a slight bigger jump and start throwing them. You won't need to pencil them but you don't need to worry about grabs yet. Get them consistent before you throw too much shit in them. Then go from there by adding your own steeze and taking them bigger.
If you're working on grabs, 90% of the problems people have while grabbing spins are caused by reaching for the grab while they're barely off the jump. Remember, get a solid pop and set your spin and THEN grab. You want to be at least 90 degrees into the spin before you start to grab and with bigger spins you'll need to wait until 180.
Remember, keep at it, and have fun. You don't need to be the steeziest kid in the park when you're learning a new trick.