Measure the distance from the ground to the top of the porch in inches. Multiply the measurement by 16 and then divide by 12 to find the necessary ramp length (for example, a 12-inch-high porch times 16 equals 192, divided by 12 equals 16). Your ramp must have a ratio of 16 inches of length for every inch of height. Measure and mark a 48-inch-wide path the length of your ramp.
Cut 4-inch by 4-inch lumber posts to the height of the porch plus 36 inches (for example, a 12-inch-high porch plus 36 inches equals 48-inch posts). Secure pairs of posts to the concrete pad or sidewalk with post anchors, according to the anchor manufacturer's instructions. Space the posts 40 inches apart (for example, use a 36-inch minimum ramp width plus space for two 2-inch-wide railings) and install a set of posts every 4 to 6 feet along the length of the ramp pathway.
Measure 2 inches below the porch on the ramp side of one post immediately adjacent to the porch and install a screw to use to secure one end of a chalk line. Tie the end of the chalk line to the screw and stretch the chalk line to the ramp side of the post at the lower end of the ramp. Hold the chalk line tight against the post at ground level, pull the line toward the center of the ramp path and let it go---the line will snap against the side posts and mark them at the proper level for the ramp side support rails. Repeat this process to mark the posts on the other side of the ramp.
Secure 2-inch by 6-inch boards to the ramp side of the posts broadside with screws, so that the top (for example, a 2-inch-wide side) of these joists falls just below your chalk line. Cut the ends of the last joist boards (for example, the lower ramp end) at an angle so that the tops of the boards are flush with the ground at the lower end of the ramp.
Cut 2-inch by 6-inch boards to 40-inch lengths. Secure these deck boards to the joists, leaving approximately 1/16-inch spaces between the boards---use nails as spacers if needed.
Measure 8 inches above the deck boards on the first and last posts and make a chalk line at this height for the kick railing. Measure 34 inches above the deck boards on the first and last posts and mark a chalk line at this height for the upper hand rail.
Cut the tops of the posts at an angle at the height of the chalk line for the upper railing.
Secure 2-inch by 4-inch boards to the ramp side of the posts just below the 8-inch-high chalk mark to create a kick plate---this will prevent wheels from rolling over the edge. Secure 2-inch by 4-inch boards to the ramp side of the posts at the top to create the hand rail.
Secure the rubber threshold plate to the board at the lower end of the ramp to create an angled transition between the ground and the first board of the ramp---a 2-inch bump would be difficult to navigate in a wheelchair.