With this project there are two main parts: a platform and a concrete footing. If you build the platform first, you can set it in place and pour the footing around it. This way you can build it on the ground where it'll be easier to reach. You want the platform to be perfectly level and square since it'll support hundreds of pounds of water.
In making the platform, first measure and cut the top (Image 1). Once you know how wide you need the top to be, transfer that measurement to the plywood with a pencil and mark the cut lines. Use a framer's square to make the corner angles accurate; this will keep the whole stand square. Then, using a circular saw, cut along the marks you've made (Image 2). This piece will be the top of the platform (Image 3). For our barrel, the wooden platform measured 30"x30".
Once you have the platform top finished, cut some 2x4s to make a frame. The frame will to go under the bottom edge of the platform, and will give the platform stability and strength as well as keep the top from sagging under the weight of the water in the rain barrel.
Once the frame and platform are cut, make the support legs. Decide how tall the stand will be and add 6". The bottom of each post will be poured into 6" of concrete to form the footer support. Use the circular saw to cut the posts to the height you've chosen.
Assemble the stand by attaching the 2x4 frame to the 4x4 posts. Use galvanized screws to do this so they won't rust. Attach the 2x4s even with the top of the posts; the platform will sit on top so they have to be as level as you can get them. If you're working by yourself, you can lay the pieces on the ground to assemble them. Otherwise you can have a friend help hold them in place while you work.
With the legs, frame and platform assembled, add some leg supports using the remaining 2x4 boards. First measure the angle and length and cut the pieces with a circular saw; then use galvanized screws to attach them. For best results, use four supports; this will make the platform more rigid. This is an important step since it will carry a lot of weight when the barrel is full; we used four 2x4s.
Dig four holes where the platform legs will stand. The holes should be at least 6" deep and just wider than the legs. Level the ground as you dig so the platform will be level as well. This is crucial so that the water barrel can't tip. Once the holes are finished, set the platform in concrete and let it cure.
Add concrete to a depth of 6" around the posts. Smooth and level the top of the concrete and let it cure. It'll cure in about 24 hours, but let it harden a few extra days before you add the barrel. You don't want the concrete to crack by adding weight too soon. Check your local building codes to make sure the project meets its standards.