You can’t always feel when you’re doing it, so pay attention to how the two feel different once you start getting corrections in class. In the meanwhile, these habits can form because of strength or flexibility issues. For strength, you’re looking at core, lats and quads, along with proper torso alignment. For flexibility, start with the glutes (and really, everything in your butt and the back of your hips). Continue to include hamstrings, calves and shoulders (you need to be able to extend your arms ahead of you and up above you).
To stretch your hamstrings, check out this image:
If you don’t have stairs handy, use anything that elevates your foot, or even do this version: