However you want. When you get further along you shouldn't sleep on your back due to compression issues, but right now that doesn't matter. I slept on my stomach till 35 weeks (I didn't get large)
Whatever is most comfortable to you i slept on my stomach all the way up to 16 weeks i am now 19w and 3 days it got so uncomfortable and plus the baby sure didnt like it, he or she would kick me.