That's about the same age I changed DD from her grobag to blanket. In her grobag she would wriggle all over the bed and end up sleeping in the weirdest positions.
I waited until summer so it would matter if she came out of her blankets and just let her get used to them. To start with she would wriggle around, but by the end of summer she started to get the hang of it and started to sleep in the one spot.
She never really got tangled in her sheets, they would just end up down the bottom of the cot all scrunched up.
HTH