So, go to a hardware store and buy a roll of four-mil polyethylene sheeting. Roll it out and cut off a patch that's big enough to fit all the way under your tent. Then, set your tent up over it. With a pair of scissors, carefully cut the ground cloth to fit the measurements of the tent. Remove the tent and cut another six inches or so off the edges of the groundsheet. What you want is a ground cover that fits under the tent without protruding at any point; if it does, it will simply trap rainwater and funnel it beneath the tent. When you're done, fold up your homemade footprint, stick it in your pack, and off you go. Cost: about $1, compared to $20 to $25 for a manufacturer's ground cover.
Now comes the hard part: Do you put the ground cover under or inside the tent? This has been the subject of vigorous discussion among Gear Guy readers, but here is the last and final word: Under. The purpose of the ground cover is to protect the waterproof coating of the tent from abrasion caused by grinding it against dirt and mud. It also helps avoid punctures from small twigs or sharp rocks. If you find that water is leaking into the tent, and so think it best to put the groundsheet inside, then you have other problems. Any modern, good-quality tent has sufficient waterproofing to ensure the floor stays dry.