The method for throwing a corked spin isn't all that much different than throwing it regular. I learned corked 7s first but you don't have to do it that way.
1)Make sure your regular spins are dialed and that when you spin them, your are tucked. Corked spins have to be tucked or you won't get your feet underneath you. Make sure you can throw a solid 5 and 7 in a tucked position, preferably with a grab.
2)Try it on a trampoline first, if you can. (Corked 5s are harder on the trampoline than corked 7s) Otherwise, wait for a soft day if you can. You'll need a decent size hit.
3)There are a couple ways to get corked on skis.
The Carved Takeoff: One way is to carve the takeoff, which works best for corked 3s and 5s. You'll want to start practicing regular spins with a carved takeoff and then gradually take them corked. If you are spinning left, approach the jump a bit from the skiers left. As you come up the lip, carve left. Set your spin with a little less pop than you would on a straight takeoff, drop your left shoulder just a bit and keep your head down. From here, it's all about staying tucked until you're back on axis and keeping your head down. Sometimes it actually helps to grab it too because it keeps you in the correct position.
The Straight Takeoff: This method, at least for me, is best for throwing anything from a corked 7 on up. Approach the lip with your feet shoulder width apart. Pop the lip and set your spin with a little more rotation than you would with a normal spin. Drop you lead shoulder just a bit and instead of keeping your eyes on the horizon, look at the run in about 100 feet behind you. (Don't focus on it though or it will stop your spin. That's just the angle that you want you head at.) From there, tuck it up, keep your head in the spin, and it will naturally bring your feet back under you. Spot your landing and you'll be good to go.
Corked spins are easy once you get them but difficult to get the hang of. Plan on taking a few diggers.
Also, if you drop your right shoulder it will tend to throw you into more of a d-spin. (in other words, more inverted.) Anyway, good luck.