Check the ground on which you are going to put the pool is completely stable. If it is not, you may have to bed the pool into a layer of compacted hardcore. For an informal pool, the deepest part of the pool, where the fish and water lilies will be, should be a rectangle or square, and it should have sides that slope outwards at an angle of about 20 degrees to reduce the pressure exerted as ice expands in winter, which is a common cause of cracks.
The rest of the pool, including the planting ledges, can be any shape you wish, but add at least an additional 15cm (6in) to the depth and to all the sides this is the minimum thickness of concrete that will be necessary to make a strong pool. In a formal pool, where it is more likely that you will want the sides to be completely vertical, you should be prepared to make the concrete about 20cm (8in) thick.
The larger the pool, the more important it is that you include heavy gauge wire mesh or reinforcing steel mesh, and it is vital that the concrete completely fills the holes and that there are no spaces left around the mesh. A very large pool will need steel reinforcing rods.