how to slip yuna to doggy her, i cannot find this button
Once you start the night, remove the blanket, move Yuna's arms to the sides and then move them again until she holds them at or above her head level.
Once done, press on her stomach area (the prompt will say "shift position" or something similar").
If you are playing on the newest update Yuna will move to her side.
If that happens, move her right arm out of the way and perform press the "shift position" option again.
If done correctly, Yuna will end up lying on the stomach.
To get her further into position move both of her legs to the sides.