Create a cardboard template and make it about an inch smaller on each side than the outer edge of the soffit vent. It will be used to mark the hole into which the body of the vent will nest. The lip of the vent will rest on the outside of the hole so that the screws will be driven into the soffit and hold the vent in place.
Position the soffit vent template. Use a tape measure to center the vent. Whether you are using a rectangular or a circle-shaped soffit vent, set it into the soffit about midway between any studs. Locate studs by looking for lines of nail heads, or by using a stud finder.
Trace the positioned template with a pencil.
Drill a hole into the soffit on the inside edge of the pencil line you created from your template. Then cut away the opening with a reciprocating saw.
Cut out any insulation blocking air flow in the attic with a utility knife. If you have blown insulation, you may have to assemble a box around the vent with wood blocking to keep insulation from clogging the flow of air.
Push the soffit vent into place and mount it to the soffit with corrosion-resistant screws. Tighten the screws down until the vent is held securely in place.