Cory cats can be a bit sensitive to salt, other than that I have had no problems but you might want to add an airstone as the water's O2 content will drop slightly. I use 2 teaspoons per gallon for treatment, (the 1 tsp/5 gal suggestion on the box is just for general usage, not treatment). Dissolve the salt in a couple quarts or a gallon of water then pour it into the tank, this prevents the formation of brine layers. Leave it in there for at least ten days at a temp in the mid 80s F.
If you add water during this time to replace evaporation losses do not add more salt.
If you do a water change during the treatment use salted water to replace the salted water you remove.
Only add salt when removing salt as it does not evaporate or get used up/broken down, if you add more you will slowly raise it to a lethal level.
You should see marked improvement in the first 2-3 days and the fish will look clear of ich in around 6-7, keep the salt in there for at least 10 days or expect it to come back.