Try to keep good form. Do not let your knee protrude beyond your front foot. Notice the front leg is at a good 90°. Make sure you step out far enough and don't tighten your stance. To do this exercise you will step out into the lunge with one leg, come back up to standing then step out alternating with the other leg... Repeat this motion...
Don't jump too high at first. Try to land on the balls of your feet. For good form here, again, try to minimize your knees from protruding beyond you feet. Push your rear end back and lean forward ever so slightly.
Modified version for beginners, just squat down, even just a little will engage your quads. Use a chair if you have to. Reach down and stand up with
your arms high.
Rock your knees up towards your chin. Keep your arms by your side on the ground. Make sure your bottom comes up off the ground for each rep. and try not to let your feet touch when you come back down. This stoping motion will engage your abs even more.