This project covers preparing and laying out the site for your patio.
Laying out a site so the perimeter is perfectly square relies on the simplest of homemade tools -- batter boards. You'll need two batter boards for each corner (unless you drive stakes close to the house). Make them from 2-foot pointed 2x4 legs and an 18- to 36-inch 1x4 crosspiece.
Start your layout by outlining the edge of the structure with temporary stakes, mason's lines, and upside-down spray paint (it sprays upside down so you can mark lines on the ground; get it at a home center). Remove the sod within the outline to make the job easier. Place the batter boards, keeping the crosspieces level (check them with a level).
Then follow the rest of the steps to lay out the site. The steps shown here pertain to patio layout, but they are essentially the same for a wall footing or walk. If you're laying out a freestanding patio, you won't have the side of the house or the doorway to use as a guide, but the rest of the steps will be the same.
About 6 hours for a 12x16-foot patio or 25-foot walk, excluding sod removal and preparation
Tape measure, cordless drill, small sledgehammer, plumb bob, mason's line, line level, wooden stakes, spray paint, rototiller
Measuring, leveling, driving stakes, making simple calculations
Prepare dimensioned plan
Scrap 2x4s and 1x4s, deck screws, masking tape, stakes
Drive a stake close to the house about 1 foot beyond your actual project perimeter. Using your scaled plan, measure and stake the other three corners, also about a foot beyond the final dimensions. (The stakes are temporary so their locations don't have to be perfect). Measure the diagonals and adjust the outside stakes until the diagonals are roughly equal. Run mason's lines between the stakes -- a couple inches off the ground -- and pull the lines tight. Then spray-paint the grass along the lines. Remove the lines, slice the sod in strips, and roll it up. Save enough to resod the edges of the project when you've completed it. Loosen the soil with a rototiller to make the excavation easy.
Reset the stakes against the house so they represent the exact location of the outside edges of the forms (1-1/2 inches more on each side than the size of the patio or walk). Tie mason's line to one of the stakes against the house and run it to the approximate center of the opposite batter board. Repeat the process for the remaining batter boards. Adjust the line parallel to the house so it is equidistant from the house along its length. Pull the lines tight and level them by driving in the batter boards. Make sure the lines touch at their intersections. Then square the corners.