When my teen was young and bedtime, I would just take him back to bed. It was constant, but repetition helped. A tip from me would be to use a nice tone of voice encouraging them to sleep in their bed. (can be hard to do when you’re exhausted).
On the odd occasion that I woke to him asleep in my bed I would get up, pick him up and place him back in his bed.
Personally I think the longer you leave it, the harder it will be.