Sign, join a game and start it as you normally do.
As you can see on the image below, ingame use the command - "!save". This command is available to everyone.
Picture 1. Command !save
After the save is done all players must type - "!rehost". Game will end and you will see the game ID that will be used for the rehost.
Picture 3. Command !rehost
In the game channel ask a member with the rank of Team captain or higher to rehost the game. The command is - "!rehost gameid" (ex !rehost 123456).
Picutre 6. Rehosting Game in Chat!
Start your warcraft3 from the RGC icon! You wont be able to view the game if you don't do that.
Picutre 7. Joining rehosted game!