Advice for doing a cartwheel?

In gym in within the next two days, I have three moves that depend on me doing a cartwheel: a roundoff, and cartwheel, and a one-handed cartwheel. The problem is that I can't do a cartwheel at all. I can't keep my legs straight, I can only go about half the way when trying to flip, and I definitely can't land them. I don't think the problem is my body type - 5'5'' and pretty slender. So do you have any advice for how do a cartwheel for someone who is completely atrocious at them?