I will show you how to make a simple bathroom in minecraft PE.
Make a 2 block high wall. (Let's just say we're in a house)
Fill it in (if your not in a house)
Add in some walls
If you need to add in a roof/ceiling
Then add in a door with 2 torches on the side with a sign that's says bathroom.
Dig a 3x3 hole in one of the corners.
Put 2 wool blocks with a cauldron and add a button then add in wood slabs around it
Put 2 quartz slabs with some stairs around it.
Put some glass panes with a trip wire hook
Put a hopper with a trip wire hook and your done!
Hope this guide helped you!