We transitioned naturally during the summer when it was too hot for a sleeping bag - so at first he was just sleeping in his nightclothes with nothing over top. I found that giving him a pillow helped him to stay more in roughly one position. When it started to get a bit cooler at night, he wouldn't let me put a blanket on him so I made sure he was wearing warmer PJs and would go in and put a blanket over him once he was asleep. For us, a cot sized quilt in the cot really didn't work - it was too small, it just got kicked off. A bigger blanket stayed put better. He went into a big boy bed at about 22 months and a single bed sized duvet being bigger and heavier seems to stay on him pretty well, although if I get up in the night to pee I do check on him that he hasn't come uncovered.