1.) On prepared firm ground (free of vegetation) make a 1800mm x 1800mm frame (inside measurement) out of 100x20 (1x4) form work.
Note: the form work is the frame that borders the concrete and stops it from spreading.
Ensure that the form work is square and level and then secure it by hammering pointed 50x50 (2x2) or 50x25 (1x2) pegs at 900mm (3ft) intervals against the outside of the form work.
Dig a trench around the perimeter of the floor area about 100mm (4") deep and about the width of the spade wide or approximately 200mm (8"). This acts like a 'footing' to give the perimeter of the concrete floor more depth and strength since it is the part of the floor that will be supporting the shed walls.

2.) Lightly hose (dampen) the pour area with water prior to pouring the concrete. Work as quickly as possible when placing concrete in form work. For instructions on how to mix concrete click here .
Start pouring the concrete into the pour area. Fill up the trench (footing) first to a height approximately 75mm (3") below the top of the form work.
Lay a rod of #3 rebar (which is 10mm (3/8") thick reinforcing steel), around the perimeter of the pour area. The rod should be at least 50mm (2") in from the form work and at least 50mm (2") down from the top of the form work.
Wherever the reinforcing rod joins, make an overlap of at least 400mm (16") and tie together with tie wire.

3.) Fill the pour area up with concrete approximately to the height of the top of the form work.
Tap around the outside of the form work with a hammer. This will vibrate the wet concrete and help it to settle, as well as getting rid of any air pockets.
Use a straight piece of lumber (the bracing piece off the form work will do) to "screed" or level out the concrete. Work the screed board back and forth with a sawing motion along the tops of the form work until the concrete is level all over

4.) Smooth over the level concrete with a trowel or float. Hold the blade flat against the surface and sweep it back and forth in wide arcs.

Tip: you will probably not get it near perfect in the first going-over, but that doesn't matter. Leave it until it dries a bit or until all the excess surface water has evaporated and then re-trowel.
A novice may need to go over it another two or three times to get a nice smooth finish, letting it dry a bit in between each smoothing.

NOTE 1: The form work can be taken off the next day ONLY if it can be easily removed. If for some reason the form work needs a lot of hammering and knocking to remove it, then best leave it in place for three days or so, until the concrete is hard enough to withstand the punishment.

NOTE 2: The concrete will now need to set (cure). Keep the concrete moist for about three days by hosing with a fine mist every now and then. This is especially crucial in warmer conditions.

NOTE 3: Concrete in direct sunlight should be covered with some type of building wrap, roofing felt or shaded for the curing period. If you must pour the concrete on a very hot day, then leave it till mid or late afternoon. Concrete sets very fast on hot days in direct sunlight and can be hard if not impossible to work with. This is especially a concern when you're hand mixing, which requires a bit of time.