Use this utility to generate mission files that you can submit to Lacuna Expanse Corp to be included in the game, in the Mission Command building.
Steps to install the Mission Editor:
Also take a look at the clients and utilities page for more goodies.
Did you know you can create multi-part missions? It's very easy. Simply keep the same filename, but change the file extension from ".mission" to ".part2", ".part3", etc. So if the start of your mission is "gun-runners.mission", then the second part would be "gun-runners.part2", etc.