As Aquarius said, focus on comfort. Be aware that being on your back can cause for one to fall asleep. I myself like to use a pillow against my back from time to time. HPS Lydia Coventina mentioned sitting on a blanket or pillow if your legs tend to hurt from sitting cross-legged.
I most of the time sit cross legged in meditation i sit like 20-30 min then when my legs feel unconfortable or hurt i sit normally. Sitting cross legged for me feels great.
As has already been answered above, the most important thing is the feeling of comfort. If you feel comfortable, you will be able to focus more easily, as being relaxed is very important for our practices.
The only thing I will add is that you must be seated during chakra cleansing and while rotating them, but for everything else the choice is yours, you may sit, lie down, and so on.
The only thing I will add is that you must be seated during chakra cleansing and while rotating them, but for everything else the choice is yours, you may sit, lie down, and so on.