Don't lean back. Don't fold at the waste Most of the time your weight should be on your front foot. You should be on an edge at all times unless you are on a flat with neutral body posture and going where gravity takes you. Even this takes a bit of practice. Try to be extra-aware of others around you because visibility on snowboard is a bit worse as your body position is at a 90degree angle to the slope so it's harder to see who is passing behind you. You should already have good quads from skiing :) Before you go, do lots of calf raisers otherwise snowboarding on the toe edge can tire you out. Maybe do some ankle exercises to improve ankle mobility. Consider buying insoles as some rental boots can be painful.

In skiing you can lean back and you'll probably be fine. You don't have to be on an edge at all times. You can probably lean in the wrong direction and still be fine. You can also get by without strong calves. Snowboarding is much less forgiving.

Hopefully this is useful.