Well I think that would depend on whether or not you're transporting the tank along with the fish. If it's a fairly small tank (say 20 gallons or smaller) you can siphon out the water until there is about 15-20% left in the tank. Depending on the size and weight of the aquarium it may take two people or you may be able to do it alone - but just move the tank to the front passenger seat of your car and put the seatbelt accross it. If the tank has a lid you also may want to tape the lid shut for the ride. When you move heavier tanks, make sure you have one hand on the underside of the tank so it doesn't break.
If you are NOT taking the tank, then you just need to purchase some fish bags from your local fish store or from Amazon. Depending on the size and how long you'll be transporting them you could put 2-5 fish in a bag and fill the bag about 1/3 full of their tank water. Capture some air for the bag and then rubber band it closed. Some people also put on a second bag that faces the opposite direction just to protect against leakage. This would last 5 fish for about 1/2 a day, 2 fish 1-2 days. Online fish stores transport these all the time - using 1-2 day shipping and I've never had one die in the bag. Some people recommend you don't feed the fish for a couple of days so all their poop is gone and doesn't get in the bag. Others put a tiny piece of sponge in the bag, I guess to collect all the poop/urine excreted by the fish - I have no idea if they treat the sponge with a special chemical to attract the poop/pee to the sponge or it just automatically picks it up. I've had other sellers just put the fish in the bag, rubberband the bag and set it in a box for transport and not worry about the waste.
Ideally the fish bags will fill the entire box. If not, then add some bubblewrap or other material to try to keep the fish bags as stationary as possible so the fish aren't sloshed around all over the place.
Another thing it depends on is how long you'll be transporting the fish. A couple fish in a bag can go for 2 days in the bag. Most sellers, however ship overnight so that the fish aren't in the bag longer than 24 hours. If it's going to be a multi-day trip I don't know what to tell you to do - you'll have to re-bag the fish so they get fresh air and fresh tank water in each bag. Then they'd likely last another couple of days.