Measure the height of your stairs. Measure your platforms. This will dictate how many platforms you will need to use. It will also give you a measurement for the length of the legs on each platform.
Calculate how long each leg will need to be by drawing the stairs on graph paper. Each stair has one leg, except for the bottom stair. Position the legs in the unsupported corner of each platform. Cut the 2-by-4s to the required length, using a saw. Set them aside.
Stack the second platform on top of the first platform with the help of another person. Trace a line across the area where the top platform rests on the bottom platform. The line should resemble an "L." Remove the top platform from the bottom platform. Draw a small line 3/4 inch behind the corner of the traced outline. Draw another line 3/4 inch to the left of the new line, and another 3/4-inch to the right of it. Repeat the process for the remaining platforms by having someone help stack and hold the platforms while they are marked.
Flip the bottom platform onto its side. Have someone match the outside face of a stair cleat to the guidelines drawn on top of the platform. While the cleat is held in place by someone else, drill two, 2-inch wood screws through the cleat to attach it to the platform. Repeat the process with the remaining platforms, except the top one.
Position the shortest leg on the underside of the unsecured corner of the second platform — the corner opposite the cleat. While someone holds the leg in place, attach the leg to the second platform by drilling a wood screw through the top of the platform and into the leg. Repeat the process and attach the next longest leg to the third platform. Continue to do this until all of the legs are attached to the correct platforms.
Lay the first platform on the ground. Put the second platform on top of it, following the guidelines so that the back of the cleat on the first platform is flush against the backside of the front edge of the second platform. Drill two wood screws through the front edge of the second platform and into the cleat to secure the platforms together. Repeat the process with the remaining platforms. After all of the platforms are attached, the stairs are complete and ready to be decorated.