Installing handicapped ramps for either permanent or for temporary use is an easy do-it-yourself project. If the person you're building it for will be up and walking soon, plywood is sufficient. For a more stable and permanent solution, you will want to use concrete. Check your local building codes prior to construction; you may need a permit.
For materials you will need plywood for the concrete forms, stakes to hold them in place during the casting, a steel trowel and a wooden float.
Step 1: Plan out exactly where the ramp will be placed. Using marking spray paint, layout the dimensions on the surface. You can can also use duct tape if you are working inside the home and don't want to be exposed to the fumes. Make sure the area where the wheelchair ramp is going to be is clean and dust free.
Step 2: Cut plywood formers to provide the outline of the ramp. From the upper level to the base level, you don't want to have a slope of more than 1 inch for each foot of the ramp in length. So, a ramp that is going to be 3 inches high will have to be at least 36 inches long. The less the angle of the slope is, the easier it will be for the handicapped person to navigate. If you are casting the concrete over a wooden subfloor, you will want to install a vapor barrier, which is available at most hardware stores.
Step 3: Stabilize the formers by using stakes or heavy blocks to hold them in place, making sure that the ramp is a minimum of 36 inches wide; 40 inches is better if you have enough room. Attach cross-members made from 1 by 2 stock at the top of the ramp and about halfway down the slope. Make sure you leave enough room to move your trowel and float freely.
Step 4: Mix up a high-grade concrete mix to a very thick consistency. It has to be able to hold it's shape on an angle. The concrete should be damp, but not wet. Trowel in the cement mixture and pat it into shape with a wooden float. Let the concrete set up for at least 15 minutes or longer before you start to do the finish floating. Wetting down the mahogany float, smooth out the surface of the concrete gently. You don't want an ultra-smooth surface, but rather one that has a sandy texture. This will give better grip to the wheelchair wheels.
Step 5: After a minimum of 24 hours you can remove the formers and, using a wire brush, knock down any rough bits of the concrete mix attached to the slope. Concrete has the ability to be physically manipulated for up to 72 hours, so don't rush the process. During the curing time, don't place any weight on the new handicap ramp. When the ramp is completely dry it should turn a slightly light gray color.