You can do this several ways.. You both can download the
Hyperlobby client and use that.. or if you want to go IP to IP boot the sim and go to Multiplayer--> Create new Server
or Join Server .. depending on which one of you is hosting the mission and which one is joining.. The one hosting has to launch the mission first when going IP to IP before others can join.
Join Server--> enter the IP address of the host :21000 ex: 000.000.000.000:21000 --> Join
Create new Server-->
Dogfight / select Max players --> Create -- select the mission --> Load
Coop / select Max players --> create -- select the mission --> Load
Campaign / select Max players create -- select the mission --> Load
You can do all this on your own as well just to see the process.. If you select Dogfight you will be in the server by yourself. After you hit load and select your base, aircraft and loadout you can go on and hit fly and your friend can still join and it will be a 1 v 1 dogfight unless you invited others to join.. If you die you can hit refly and come back to the field of battle.
In a coop and a campaign it is different. First off there will be AI so any slots that you see that are not filled with a person will be flown by AI.. and in a Coop and a Campaign the host cannot hit fly until whoever is a joining the server has joined. If the host does this before the person joins they will not be able to get in. If anyone dies they will not be able to come back until the host relaunches another mission. If they stay connected and the host ends the mission as long as the guest does nothing when the host relaunches the guest will still be there.
Others will not be able to join uless you give them your IP.
Hope that helps.. welcome aboard.. See the Nugget's Guide in my sig.. Hyperlobby is the shiznit.. b ut if youwant to go IP to IP until you get more comfortable that is understandable and cool too..