Hope none of us will ever need it, but just in case, here is some info on how to do it: http://www.mchportal.com/fishkeeping-ma ... -fish.html
The photos show a cichlid but we have tried the method on larger plecs successfully, particularly when administering medication (always more effective when taken orally, particularly if the fish are not eating at the time).
The method is to be used as an ultimate means of saving a fish, as indicated in the article. We use it when the fish appears to be a "lost" case, unless action is taken. It works, it just needs a bit of care. If a fish is about to pass away the only thing that might help it is intervention from the hobbyist.