Prepare the old concrete by sweeping, washing and removing any loose chunks and chips. Fill in minor cracks with concrete epoxy.
Build a concrete form around the existing concrete with 2-by-8s. Extend the top of the 2-by-8s above the surface of the concrete to the width that you want your overlay to be. Place the form boards the same distance away from the edges of the existing concrete. The overlay should be at least 1/2 inch. For thinner overlays, you may be able to trowel on the overlay without the use of a form.
Check the angle of the corners with a square and use a level to make sure the boards are even. Create a form that is slightly uneven for a pad that needs water runoff. Secure the form by packing soil behind the boards and putting two stakes at each corner.
Mix an overlay mixture that is 1 part cement to every 2-1/2 parts sand. If you want stones in your overlay, use 3 parts rock to every 2 parts each of cement and sand. Use a polymer-modified mixture for extra protection against environmental factors. Follow the instructions of the specific manufacturer for how to apply polymer overlays.
Coat the interior side of the form boards with vegetable or another type of cooking oil for easy release. Coat the old concrete with cement paint (a soupy mixture of cement powder and water) or with a liquid concrete bonding agent.
Pour the concrete into the form. Use a two-by-four that is cut to extend just beyond the edges of the form to level the concrete. Do this by pushing and pulling the board back and forth (like a saw) as you move it across.
Use a metal float to smooth the surface. Skate the float on the concrete by carefully and lightly pulling it across the surface in arcing movements.
Slice the concrete 2 inches deep along the inside of the form boards to help with final release.
Create a surface for the pad, if desired. Pull a wet broom lightly and straight across the entire surface to create a traditional treaded pad. Use a concrete edging tool to smooth down a border for the pad. The edging tool will also round the edges and help prevent crumbling. Etch or stamp into the wet concrete to create decorative patterns or to suggest the appearance of natural stones like cobblestone or slate. Concrete overlays can also be stained or tinted. Many polymer overlays are also decorative and made to have different looks and colored finishes.
Cover the pad with plastic and let it cure for two days. Spray a concrete curing compound onto the new concrete to ensure consistent and proper curing. While the overlay cures, keep it wet by periodically spraying it lightly with water. Remove the form boards. Allow the overlay to finish curing. Coat with a concrete or polymer overlay sealer.