Mark the fence line with stakes and mason's string. Place a stake on the outside of the corner with a hammer and connect the corner stakes to the previous stakes with mason's string. Place another stake 3 feet out from the corner stake along the line, using a hammer.
Mark the adjacent side by positioning the end of a tape measure at the outside corner stake and out along the adjacent, connecting side. Open the tape measure to extend beyond the 4-foot mark and lock it in place, pressing the lock button.
Position the end of a different tape measure at the 3-foot stake on the first side and open it past the 5-foot mark. Lock it in place.
Angle the second tape measure toward the first, intersecting them at the 5-foot mark. This gives you the diagonal measurement. Angle the second tape measure while it's still intersected at the 4-foot mark for the perpendicular measurement.
Run a mason's string from the stake the two tape measures are at to the outside corner stake. The 3-foot and 4-foot mason's string forms the right angle for the fence. Extend or shorten the mason's string if needed.
Stake out the exact dimensions for the rest of the fence according to your map.