Measure from the top of the doorjamb to the ground using a measuring tape. Record the measurement in inches. Multiply the total by 12 to determine the rise of the steps. In this example, the top of the doorjamb measures 7 inches tall, so the rise is 84 inches.
Cut a piece of plywood measuring 84-by-38-by-1 1/4 inches using a circular saw. Doorjamb widths may vary, starting at 30 inches. In this example, the width is 34 inches. Sand the edges of the plywood with medium-grit sandpaper. Paint or stain the wood as desired. Let the plywood dry overnight.
Cut two 7-by-4-by-4-inch posts to support the top of the ramp. Cut one end of each post at an angle of 8 degrees. Place a protractor against the end of the post to determine the correct angle. Sand the cut edge with medium-grit sandpaper.
Place the two posts on the ground spaced 38 inches apart with the angled edges up and facing the same direction. Apply a thin layer of wood glue to the angled edges of the posts. Rest the plywood piece on the posts. Ensure the edges of the plywood are square with the edges of the posts. Hammer one 4-inch nail through the top of the plywood into the center top of each post to secure it.
Place the ramp over the steps with the posts flush against the garage wall and the ramp bottom resting on the ground.