Learning to wet while laying down is essencial to wetting the bed in your sleep.
At first I found it difficult. I needed to raise myself on my elbos to pee. Then It became easyer and I can now do it laying down, in any position. It's a trainning and it take time. Then, if you want to do it in your sleep, you have to trust your diaper and set a few conditions right.
Make sure you go to bed without feeling the need to pee.
Drink about 2 good glasses of water right before going to bed.
You should be able to fall asleep fast enough without being aroused by your diaper.
If you are relax enough, you will at first wake up just enough to feel the need to pee and release.
The process take time, but eventually, it will do it by itself during your sleep.
It's been at least 2 good years since I sleep in diapers and I often wet in my sleep now. If I don't wear at night, I wake up on the verge of wetting my bed.