Always place baby on their back at the foot of the cot and if using blankets, firmly and securely tuck these in under the mattress. By positioning the baby this way, they physically can’t move down the cot any further (as they are already at the bottom of the cot). This feet at the bottom of the cot position will reduce the risk of baby slipping under the blankets.
Aim to have the blankets no higher than underarm height and use light weight blankets (preferably breathable) in layers that can be added or removed easily according to the room temperature