Position the ladder on a level surface, making sure it does not wobble. Then climb up to the roof.
Measure the attic vent to determine how much hardware screen is needed. Examine the vent style. Some allow the hardware screening to be tucked underneath the existing vent edges. Others will require that the screening be nailed to the roof.
Cut a piece of hardware screen or mesh to the size needed for the vent using tin snips. Leave 1 to 2 inches of screen overlapping on all four sides to secure it to the vent or roof.
Secure the screen or mesh to the roof with six to eight long nails; they need to be long enough to go through the roofing to attach to the wood below. Place one at each of the four corners of the screen. Use the remaining nails to attach the mesh between the corners.
Tug gently on the screen to make sure it is secure. Place your finger around the perimeter to ensure no gaps exist large enough for a bird to wriggle through. If you find any, secure the mesh with more nails.
Repeat as necessary with additional attic vents until all are protected.