When your toddler is about 15 months old, put a mattress in their room. Introduce the new bed slowly and do more and more activities on it as the days go by. Start with daytime naps and then later add nighttime sleeping on the new bed.
After a few days start putting them down for naps on their little mattress. Once they have mastered naps on the bed, then take down their crib and begin nighttime sleep on the bed.
When you begin actual sleeping on the mattress be prepared that your child may cry or show anxiety. Put your child down with the same routine as usual. If they cry when you leave, give them a minute or so to see if they’ll figure it out.Go in every few minutes to calm them and return them to their bed until they have settled down to sleep. In our family, it only took 2-4 comforts until they settled. It only took about 2 days until they didn’t require any additional comforting besides the initial tuck-in. Whenever you find them asleep on the floor, gently put them back into their little bed. If they fall asleep in front of the door (so you can’t open the door) then let them sleep until they wake up and then guide them back to bed. If they wake up in the middle of the night- comfort them and put them back onto their mattress. After a few weeks their little bodies will orient themselves and be able to stay on the mattress without conscious effort.