Click Host Game and click Next.
Choose a name for your saves if you want but remember that if you want to have multiple playthroughs with different people at the same time, you will need to change this name to something different and remember it. Enter your name for your character you'll be playing and click next.
Depending on which method you have used, you will need to either give out your real IP address or your Hamachi IP address to your fellow players.
You will then get to a window that will list all players in the current session on screen. Think of this as a 'Lobby' like Left 4 Dead. You will need to tell people that you are waiting for them to connect as this is the time to do it.
When all people have connected to your Lobby, Press Next.
You can either Start a New Game or Load a previously saved game. Make your choice and away you go.
When you start a New Game and don't see your friends next to you on the train, don't panic! You will not meet up with your friends until you get onto the Von Braun. You are free to explore the town on street level, find any hidden secrets, do the tutorials and do your Yearly Postings in which ever career you choose.
After your 3rd year of training, the game will attempt to 'synchronize' with your friends. If your friends are not up to their 3rd year of training by this point, you will be stuck on a temporary 'Synchronizing...' screen until they are all ready.
When everyone has completed their training, you'll all see the starting cutscene and then awaken upon the Von Braun.