When you lay out the perimeter of a fence, you can do it the traditional way, using strings, or -- if you're a techie -- using a laser. Either way, the very first step is to determine your property lines so you can stay inside them -- unless you live in a state that allows shared fences on the property line, and your neighbor agrees to the project. If you skip this step, you might have to take the fence down, perhaps after a few years when you least expect it. Once you've squared the fence corners and set the markers for the post holes, be sure to call the utility company before actually digging them to make sure you won't hit any underground cables, wires or plumbing pipes.
Remove any vegetation in the path of the fence. Dig up bushes and clumpy plants by the roots to prevent them from growing back after the fence is built. Get permission before clearing anything growing on a neighbor's property.
Choose a corner of the house, or some other permanent landscape feature, for your starting point. Walk out the rough perimeter of the fence, using a tape measure to measure distances, and mark the positions of the corners with chalk.
Construct batter boards by screwing one 1-by-2-inch stake to two others so that when you pound the two stakes into the ground, the third one extends horizontally between them. You'll need two of these for every corner except the one where you started. Temporarily install one batter board behind each chalk mark, one facing along one side of the fence and one facing along the perpendicular side of each corner.
Attach a mason's string to the landscape feature you used as a starting point and stretch it from there to middle of the batter board that faces it. Extend a piece of wood perpendicularly from the center of the batter board, and adjust the batter board to align the string with the piece of wood. If you're using a laser, shoot the laser along the board, and adjust the orientation of the batter board until the laser and wood extending from the batter board are aligned when the laser falls on the starting point.
Extend another piece of wood from the center of the other batter board at that corner. Adjust the batter board so the two lengths of wood intersect on the chalk mark, and then make another adjustment of the batter board to make the angle between these intersecting pieces of wood 90 degrees when you measure with a carpenter's square. The corner is now squared; pound the stakes into the ground. Square all the other corners in the same way.
Stretch mason string between the centers of all the batter boards to delineate the perimeter of the fence. Measure the positions of the posts with a tape measure, and drive a stake at each point. The outside edge of the stake should just touch the line. If you're using a laser, shoot it from the center of one batter board to another while an assistant sets stakes so their outside edges just graze the beam.
Center each post hole on the stake when digging it. Leave the string in position, and set each post so that its outside edge just touches the string when it's level. Alternatively, use a laser to double-check alignment when you're setting the posts. The laser should graze the outside edge of each post, meaning when the post is slightly tilted to the outside, it intercepts the beam.