Frame your roof as directed by the building plans, if starting a project from scratch.
Count the number of rafters to determine the amount of lumber you will need. Rafter ties are required every 4 feet on center by Section 803.2.1 of the International Residential Code, meaning that you only need one tie for every other 24-inch on-center rafters or for every third pair of 16-inch on-center rafters.
Cut the rafter ties to the appropriate length using a circular saw and angle the ends so that they fit the slope of the roof.
Install the rafter ties. The ends must be placed against the rafters leaving a 1/2 inch from the rafter's edges.
Use a level to double-check that it is flush before installing permanently.
Nail the rafter tie into place using three 10d nails, on both ends.