You should sit on a desk, your back will hurt even more if you keep drawing laying in bed or couch. Believe me, it's the worst choice you can do for your back.
Sit on a desk, better a not too low one, and get a good chair (get a typical office one if you can, those ones where you can regulate the height...).
If you have a low desk, and you can't get one of those drawing desks that you can incline... you can do what I do.
Get a plywood board where you will put your sheet and fix it to the board with adhesive tape. I suggest to use this tape because you can remove it without damaging the sheet.
Put a big book under the upper side (I use a book by Terry Brooks that I never read
).
In this way, you will charge less your back and neck because your back and neck is more straight. And you can draw basically on all desks.
This is what my "portable drawing system" looks like
:
Cheap but efficient, at least for me