Currently you can’t create a brand-new map, but the sheer degree of change you can inflict upon an existing one makes that reasonably academic. You also can’t create campaign maps just yet, but your offering will be playable in multiplayer and skirmish. On the first run you’ll need to show the editor where your Sup Com executable is. Once that’s done, head to File > Open and choose the “SCMP_0022 – Concord Lake” map.