If it's from one tank to another, put him in with only a couple other fish into a 5gal bucket or other kind of bucket, like a water change bucket, and take some extra air hoses. Tie three knots loosely in the line, and with the bucket on the floor underneath the tank, siphon a little until water is running into the bucket. Tighten the knots until it's just dripping, leave it for an hour, without AC blasting on it or whatnot. This is only if you really care about your fish, or you paid a lot of money.
Otherwise, put them in a bucket or container, add tank water every 10 or so minutes for half an hour with a measuring cup or something. Three times, a lot easier. It just prevents them from getting any kind of shock from a sudden variation of levels.