Concrete projects such as patio building need a good form to hold the concrete as it's poured. The most common form type is a frame made of lumber. Variations on this basic method are used for different types of concrete pours.
Excavate the area for your concrete pour to a depth that is the same as the planned slab thickness, plus 10 cm (4 inches). Compact the soil within this area firmly, and add a 10 cm (4 inch) layer of finely crushed gravel.
Use dimensional lumber laid on edge to line the sides of the excavated area. Select lumber that is the same width as the thickness of your slab. For example, if you are pouring a 15 cm (6 inch) slab, use 5cm x 15 cm (2 x 6 inch) lumber.
Nail the lumber together at the corners.
Drive wooden stakes into the ground outside the form, spaced every 60 cm (2 feet). Place the stakes directly up against the form to provide additional support when the full weight of the concrete is poured.
Spray the inside face of the lumber liberally with a vegetable-based non-stick cooking spray so that the cured concrete will release easily from the form.