As of March 2015, Lowe's and THISisCarpentry have instructions on their websites for building handicap ramps. Both sites explain how to determine the correct slope and size for specific houses. They discuss when L-shaped ramps are appropriate and when to build U-shaped ramps.
Lowe's has a 15-step set of instructions for ramp construction. The directions begin with a list of tools and materials necessary for completing the job. It also provides diagrams that show many phases, and some are labeled with measurements.
THISisCarpentry includes both drawings and photographs to illustrate the building process. The photos display the construction stages of an actual ramp. The site also offers recommendations for appropriate materials, such as pressure-treated wood.