Having never built a mission I won't be able to tell you how. There is good info here in the forums and in your CFS2 docs. on how to build missions.

What I do know is that when you do get a mission built, others will have to have the mission file in order to fly it with you online. You either have to make it available via download from somewhere or directly send it to your friends. Everyone that wants to fly that mission together, or alone, MUST have the mission file.

Hope this helps.