Well, I can't really give you any advice 'tailer-made' for your body but I can explain how to start your spin. Also I don't know how much 90 pounds is as I live in (/on continental) europe...and we don't use pounds, we use kilo's.
I'll explain how I start backspins when not doing it from a 6-step or something.
I go counter clock wise!
1. Lie on your back, facing the ceiling. Lift yourself up on your left elbow. You'll be lying slighty on one hip/bum (left side) cause you're leaning on just one elbow. Keep your left leg like it is, that is stretched. Tuck your right leg in to the right. Check out my mighty good drawing to see what I mean by this

...you could have your right hand on the ground next to you (right side) for some balance.
2. Now, swing your right leg over your left. Try to keep your right leg as close to the ground as possible. When your leg can't go any further (stretched out) start collapsing on your back (stop leaning on one elbow, start lying on your back).
3. At that same moment, you'll notice that your right leg wants to go up instead of further along the ground. Also your left leg wants to follow the movement. Kick (gently) your left leg to the side (NOT UP!). So at this stage, your right leg has moved in a circular motion from the right to the left and is now moving slighty up (IN A CIRCULAR MOTION, SO NOT STRAIGHT UP!) towards your head. You left leg is kinda following your right leg, first along the ground, then up and towards your head.
4. Once both legs are above your head, you can grab your legs and start the actual backspin. It might feel unstable, spinning between your shoulderblades but keep practising balancing until you feel like your weight is devided 50/50 between your shoulderblades.
5. If you find it hard to get ON your shoulderblades (you're still spinning on your lowerback) put more strength in the circular motion TOWARDS your head, kick it up if you will. You can also pull your legs to your head, like I said before.
I tried to attach a vid of me doing a backspin but it didn't work.