What is the best way to put baby to sleep?

My babies (twins) are 6 months old, and yet don't sleep 6 hours straight? I bath them at 7:30 and i put them to bed by 8:00-8:30. But the problem is i have to be there, or sometimes they only sleep on their bouncers. What am i suppose to do? I don't sleep at all. Maybe 3-4 hours the entire night? Should i let them cry. They roll-over so much.