Here's what you need:
Block of quartz
Step 1: Building the Small House Thingy
To start your toilet off, build a small structure. The dimensions I'm using are 6X6X4 or, 6 blocks wide, 6 blocks length wise, and 4 blocks high. I'm using quartz, but you can use whatever you want
Step 2: Lighting the Structure
Put torches around the walls, so you don't have to poo in the dark :3
Step 3: Flooring
For the flooring, you need quartz blocks and slabs. Cover the entire floor with quartz SLABS. Then, in the second picture, destroy the one where your doorway is going to be. Destroy the one straight up from it too. This is because if you didn't, you wouldn't be able to get in. Then, dig one block down and replace the blocks dug with quartz blocks
Step 4: Making the Base
First, destroy 3 slabs horizontally, then, destroy one vertically in the middle. Dig ONE block down in the very middle (see picture 2) Then, replace the duggen out block with a quartz block.
Step 5: Base 2
Now, take your quartz stairs, and place them on the grass blocks (or whatever you're building on top of) Make sure the front of the stairs are facing you when you place them. Now you should have no grass blocks showing (picture 3)
Step 6: Finishing It Off
Now add water to the tiny hole. Put a trapdoor on the block above the water. Open it and it can be used as a toilet seat. You've officially created a toilet in Minecraft! Hope you enjoyed!