Ok, this is a rough approximation. I can work out a step by step process later including the exact wording of the Messages tonight when I get home from work. I intend to do that anyway so I can get it posted in the PBW FAQ.
A TCP/IP game is at it's root a Simultaneous turn game identical to a PBEM or PBW game. The files used are the same, a .gam file for each turn, a .plr file for each player, and some other turn log files and text files. The TCP/IP has a built in automatic file transfer utility that uses TCP/IP protocol.
I will assume you know how to set up and play a TCP/IP game, and are familier with the care and feeding of a standard PBEM game to begin with or you wouldn't be asking this question.

I will skip those parts.
Once you have played the turns TCP/IP and are ready to convert to PBEM, all the players simply exit the game. In a simultaneous game the turns are saved as you end turn so you don't have to worry about saving or anything.
Each player and the host will now have current .gam file on their machines. Assuming they didn't drop out while it was still in the process of transferring them. If that is the case then the host player for sure has the current .gam file and he can email it to all the players, along with any move logs, combat logs and text files.
Each player can then at their leisure open up SEIV and load the game up on their machine. When they do they will get a prompt asking if they want to continue the game in TCP/IP host mode or somesuch. I don't remember the exact message as it's been a few months since I did this. Answer no to that prompt and it will take you to the empire selection screen that you normally see when playing a turn on a PBEM game. Here each player enters their password and selects their empire. Play once in the game turn is identical to either TCP/IP, PBEM, or PBW. Once you are done and hit end turn you go to a TCP/IP game screen. Simply exit this screen. You don't have to do anything. Go to the Savegame folder and you will see that there is now a new .plr file for whichever empire you just played. Email that to the game host.
As the game host when all the .plr files are in do the same steps you would normally for a PBEM game. You will get the same "Do you want to continue game in TCP/IP mode message". Answer no and log in as host with the game master password. Hit end turn and the turn will process. Once done email the new files to the players, rinse and repeat as neccesary.
One nice little trick is if you use Matryx's mod launching program you can launch right into your turns and won't get the offending "DO you want to continue in TCP/IP" Messages or the TCP/IP screen at the end. It will just let you play.
And to beat all if you set up a game as TCP/IP, play a few turns via TCP/IP and then a few as PBEM, you can return later and play turns via TCP/IP again if you can get everybody together. To do this you simply use the current turn and load and join the TCP/IP game as normal, answering yes to the TCP/IP prompts this time.
Geoschmo