Measure the area and the chairs. A stadium platform should be large enough for several chairs, a couch, or a loveseat and a few chairs. Depending on the seats you choose, use a tape measure to determine the depth and width of each seat. After measuring the chairs, measure the area they will occupy to be sure there's enough room for your front row chairs and a stadium seat platform.
Calculate riser width and length. Use the seat measurements to start your calculations as follows; total seating width plus 20 inches for the width of platform, by depth of the largest chair plus three feet for the depth of platform. A three-foot allowance on depth allows the chairs on the lower level to recline fully. It also allows plenty of walking space so guests don't have to climb over each other to enter or exit their seat. So, if your seats are 24 inches wide and 20 inches deep and you want three seats on the platform, the platform would need to be 6 feet, 20 inches wide by 3 feet, 20 inches deep. The platform should be 18 inches tall. That height will allow adequate viewing over the people in the lower level. Divide the total riser width by four to figure how many 4-by-8 plywood sheets will be needed.
Cut the plywood. On the four-foot width, cut the plywood to match your platform depth. Continue cutting additional plywood sheets until, sitting next to each other, they cover the width of the entire platform.
Cut the 2 by 4s. Use the circular saw to cut the 2 by 4s into 18-inch sections. Cut four 18-inch pieces per plywood section. These will be the legs. Measure and cut additional 2-by-4 pieces to match the width and depth of each section of plywood. These will be the supports. You will need two sets of supports per section of plywood.
Assemble each section. Attach the cut 2-by-4 supports to each piece of plywood using screws. The supports should be attached by inserting screws through the plywood and into the narrow two-inch side of the support board. Place screws every six inches. The supports should look like a box on the underside perimeter edges of the plywood. Take the leg pieces and screw them in place in each corner of the support box. Repeat assembling a second support box on the end of the four legs, framing and supporting the end of the legs.
Cut the 2-by-4 beams. Turn the riser over. Measure the length and width inside the supporting 2 by 4s of the riser platform. This measurement will tell you how many beams to cut based on spacing 16 inches on center. Cut the 2 by 4s to the inside-measurement of the supports. Place the beams at the 16-inch intervals and use screws, at six-inch intervals, to attach them to the supports and to the plywood.
Attach the riser sections together to form one platform. Take the four-foot riser sections you've assembled and attach them together with screws. This is accomplished more easily when the risers are turned upside down. When finished, turn the entire platform into the upright position for finishing.
Finish the riser. The finishing process depends entirely on the purpose of the riser. If this is an outdoor project, a weather-proof outdoor stain works nicely. If this is an indoor project, paint or carpeting can give a nice acoustical effect. You can also add 18-inch strips of plywood to enclose the legs before carpeting.