It's all about putting pressure on one part of the ski or the other. For ollies you pretty much just bring your toes and feet up to load the tails and pop off that tension. Nollies, you lean forward a bit and pop off the tension on your noses. For butters, you sort of spin while leaning super far forward or backward. All take practice. Ollies/nollies are really easy, but butters will take some more practice unless you're heavy/really strong. You just have to try and see what works for you.