Intro
Laying out a site so the posts are in line and the corners are perfectly square relies on the simplest of homemade tools -- batterboards. You'll need two batterboards for each corner (unless you can't drive batterboards close to the house). Make them from 2-foot pointed 1x2 or 2x4 legs and an 18- to 36-inch 1x4 crosspiece.
Start your layout by driving batterboards perpendicular to each other and about 3 feet beyond the outside corners of the site where you have removed the sod. String mason's line tightly between them and fine-tune the batterboards so the crosspieces are at the same height. Drive in the legs so the crosspieces are level (check them with a torpedo level).
Then follow the steps illustrated on this page to lay out an attached deck, beginning by plumbing the ledger location to establish the center line for the outside posts. If you're laying out a freestanding deck, you won't have a ledger as a guide but the rest of the steps will be the same.
Prestart Checklist
Time
About six hours for a 12x16-foot deck, excluding sod removal and preparation
Tools
Tape measure, cordless drill, small sledge hammer, plumb bob, mason's line, line level, wooden stakes
Skills
Measuring, leveling, driving stakes, making simple calculations
Prep
Remove sod
Materials
Scrap 2x4s and 1x4s, deck screws, masking tape, landscape spikes