There's mad tutorials on youtube. It's easiest if you have photoshop.
Basically, you make whatever image you want to use black and white then mess with levels/contrast until you have enough difference to make it viable.
Make sure you have bridges so that none of your uncut parts are not attached to the rest of the paper. And don't use card stock, shit is thick. Go to the art store and buy a sheet of Mylar. It's like $5 and thinner and more durable than cardstock.
Make sure your exacto is sharp. That's it. You might fuck up the first one you try because it can get confusing when you're trying to figure out what to cut and what not to cut. But it ain't too hard.
Good luck.