Check underneath the porch for any existing animals. You don't want to lock any animals under the porch once it is sealed off. If you are concerned about dealing with any animals on your own, call your local animal control, wildlife rehabilitation agency or pest control company to have them removed.
Measure the height from the ground up to your porch. This measurement will be used to determine the height of the lattice fence and screening that will be used to keep the rodents out. Add 3 inches to this measurement.
Dig a 3 inch deep trench underneath the porch. The lattice fence will be buried to help discourage any animals from digging underneath it.
Cut the lattice to fit your measurements. Don't forget to add the 3 inches for the buried portion of the lattice. Use a hand saw or hack saw to cut the wood to fit.
Attach the mesh screening to the inside of the lattice work. Make sure that the mesh has holes that are no bigger than the diameter of a pencil to discourage animals from crawling through. Use a staple gun to secure the screening to the lattice.
Place the lattice into the trench and bury it under the dirt. Scoop the dirt back into the trench and pack it down to secure the lattice.
Attach the top of the lattice to the porch. Using a power drill and screws, attach the top and sides of the lattice to the wood on your porch. Place a screw every 2-3 inches for a secure fit.