If the skunk is currently living under the porch, you might get sprayed while putting the powder under the porch. Since they're nocturnal (unless rabid) they tend to abhor bright light. If you have access to a flood light/work light, place it so that it's shining under the porch. It should abandon the porch relatively fast. If there's not an easy way to flood the area with light, then grab some flour from the house. Sprinkle it around the porch and keep checking it every 20 minutes or so after twilight. Once you see tracks in the flour leading out from under the porch, that would be the time to sprinkle the urine under the porch. You'll probably want to setup lattice, or some other barrier, under the porch to keep it from getting back in. Make sure to bury it since they can dig.