Intro
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.
Prestart Checklist
Time
About 6 hours for a 12x16-foot patio or 25-foot walk, excluding sod removal and preparation
Tools
Tape measure, cordless drill, small sledgehammer, plumb bob, mason's line, line level, wooden stakes, spray paint, rototiller
Skills
Measuring, leveling, driving stakes, making simple calculations
Prep
Prepare dimensioned plan
Materials
Scrap 2x4s and 1x4s, deck screws, masking tape, stakes