Shipping the fish should be simple, so long as you plan to be at your new location before your fish get there. You should mail them out the morning before you go, if possible. Otherwise, mail them out the day before or have a trustworthy friend send the fish for you. USPS will come collect for you, I believe.
I've never sent out fish, but I've had fish sent to me. Most of the sellers get styro boxes, line the boxes with newspaper, add heat packs, another layer of newspaper. The heat packs are usually lined against the walls. Then the sellers bag the fish in either regular bags or in Kordon breather bags which are really cool. Most of the fish I've received have come individually bagged. Then newspaper is stuffed around the bags to prevent too much jostling or any rolling bags. Styro lid is set down and taped so it is secure. That box goes inside another box and is then sent off to the post office.
Others may have more detail about how much water to add, but the process is pretty simple. If you want, there are several friendly sellers on aquabid and the like, so you could probably contact one of them for more information.
Oh, I do recommend double bagging the fish if you are using standard bags.