First, you need to make the installation of sewer pipes, and take into account all the pipes that will be under concrete. And only then begin to work.
For this we need the 4 pegs. To hammer the pegs into the floor at each corner, and cut around the thread.Then using the water level draw a dash on pegs and align the thread on dash.
Now we need to make the diagonal of the same threads for bias.
Harvests concrete solution in the proportions, 3 shovels of sand, 3 shovels of gravel, 2 shovels cement. And concreting the bathroom makes the slope, using any instrument which will be useful to you.
That's all. Concrete leave to harden for a few days. Good luck.