This is what I did with a 20 gallon:
Buy some plastic storage boxes with lids, square with flat bottoms, and fill them with your tank water. Take about 50% or more if you can manage it and empty as much of the rest as possible from the tank. I left the gravel in the tank and wrapped it in towels.
Bag up the fish and put them in a polystyrene box to keep them at an even temperature, then cart the whole lot to its new home. Set up the tank first, empty the preserved water into the tank, using a plate on the gravel for less disturbance, and plug in the heater. Float the fish bags on top for half an hour then let the fish out. Fill the tank with fresh water of the same temperature, with water conditioner, as if doing a water change. Turn on the filter and leave everything to settle.