1. find a big pair of stretchy stirrup tights or pants, or just any sort of very large hosiery.
2. take one leg of the garment and shove both of your legs through it, up to your knees.
3. stick your head and shoulders through the other leg
4. lean back, the garment will encase you (if it's big enough) with a little opening in the back.

The variation of this is to

1. put one leg of hose over both of your legs while keeping the free leg behind you.
2. if you can contort yourself enough (or if the hose is stretchy enough) pull the other leg over your head and shoulders and chest from behind you.

Yet another, more constricting bodybag:

1. get two pairs of large, stretchy hose.
2. put your left leg through one leg of one pair of hose
3. put the other leg of the hose over your head and shoulders.
4. get the other pair of hose, and put your other leg through one of its legs
5. put the remaining leg of hose over your head and shoulders.
6. lean back, and you've just wrapped yourself tightly with glorious, shimmery, beautiful stretchy stuff...

If the hose is large enough, you can put both of your legs in each leg of hose instead of just letting your legs go free.