Skiing uses very different muscles than boarding. You should get on the HO website and check the "standard" setting for the bindings and ensure they are correct. It may be set up wrong. If those are correct, I recommend speeding up more to at least 30. Your ski will "feel" better the faster you go. No more than 34-36 though.
Knees slightly bent, not locked, back straight and shoulders back. Arms in close to the vest. Slalom ski design makes it perform best on an edge. You may feel unbalanced when skiing in a straight line behind the boat. Unlike a wakeboard, that is stable when running flat.
Practice, practice, practice. Just like everything else.
Warning, don't try to run a slalom course unless you want to obsess about getting better.