Place 2 blocks of any solid material side by side for the base
Put a stair block on top of the back block facing forward for the toilet seat
Place a trapdoor on the front edge of the stair if desired for the seat lip
Put a block behind the stair for the tank
Add a button or lever on the tank for a flush handle
Place a water source block inside a hidden hole behind or under the toilet
Cover the water with a slab or trapdoor to keep it hidden
Add white blocks around the toilet for a cleaner look
Place a sign or item frame nearby if you want extra detail
Build a small enclosed room around it if you want a bathroom setup