Put down a large sheet of plywood over the floor joists in your attic to give yourself a place to stand while you work.
Place a rafter vent against the ceiling, between the rafters at the point where the ceiling meets the floor of the attic. The rafter vent will prevent the insulation from blocking air flow from the soffit vents; it should be positioned so that the two side edges are flat against the ceiling, leaving a small gap between the ceiling and the middle portion of the vent. Use a staple gun to secure the edges of the vent to the roof decking.
Install additional rafter vents between each rafter in the area of the attic where you are planning to insulate.
Add insulation between the floor joists in the attic, making sure it goes all the way over to the eaves. If you are using insulation blankets or batts, simply roll the insulation out until it rests securely against the rafter vents. If you use blown insulation, place a piece of foam board in front of each rafter vent before blowing the insulation into the eaves. This will prevent any insulation from going into the vents and blocking them.