There are a number of different finishes to consider depending on the purpose of a D.I.Y. concrete slab.
Smooth concrete can be slippery when wet. For an anti-slip footpath, wait half an hour after laying the concrete and brush over the surface with a broom in one direction.
Edging tools can be used on the corners of the slab for a professional finish and to assist in draining water away. Use a concrete edging tool after screeding the slab but before trowelling the surface.
Place the edging tool along the formwork and lower the flat surface onto the concrete. With the front or back of the tool slightly raised, move the tool along the side of the slab moving forward and then backwards.
As you do this you might encounter areas of large aggregate stones. Use the tool to push these into the slab, and then go back along the edge to smooth it out.
A trowel can be used for a smooth finish. Let the slab set for half an hour and then smooth out the surface with large sweeping motions.
Repeat with the trowel at a slightly higher angle until the desired smoothness is achieved.