A detailed breakdown of the process is available through the Community Wiki, but the quick-and-dirty version is that you'll select and arrange assets through the editor, save them as a custom composition, and select the icon to publish it to the Steam Workshop. Players will also be able to share custom compositions from Arma 3 directly to the Steam Workshop following the 2.0 update.
Experienced Arma players may also recognize a few surprises from previous versions of the game, Bohemia teased, 'if they know where to look and what to do.' The firing drill, called Course of Fire: Gray, is a challenging run that offers a gold medal to players who can complete it in under two minutes.