Clear the decks and clean the existing concrete.
The area needs measuring off and shuttering to stop the concrete running everywhere and give a clean, straight edge. I am using 2x2 rough sawn pine
If you were starting from scratch then dig down about 2" over measured out area and put the shuttering at ground level.
Hammer a stake of wood into the ground until 2" is left showing and screw a long rail to it.
At the opposite end of the rail hammer in another stake, using a spirit level on the rail raise or lower the free end and once level, screw it to the new stake.
I then repeated this going around to form the rectangle, using a large set square to ensure my first short rail was at 90 degrees to the long rails.
Now the fun bit, before fixing the final corner you need to measure diagonally from corner to corner. Move the free end of the short rail until both diagonals are the same and voila, one perfect rectangle, or square as needed.
Tomorrow the concrete arrives and lucky me gets to mix and move a ton of the stuff, fill in the area up to the top of the rails, wait a few days and with luck it should be what we want.
Pics to follow