1) World Map
Sketch out a map of the world you are invading, divided into regions so that it looks like a Risk board.

2) Invade!
At the beginning, all regions are controlled by the aliens. Pick one region to drop into and assault. Play mission.

3) Enemy Movement
At the end of the mission, if the PCs are successful (i.e. reduce threat tokens to 0) they control the region they attacked. Also roll 1d10 and apply the following modifiers:

+1 if the mission was successful
+1 for each Strength used during the mission
+1 if the PCs control 50% but less than 75% of regions over the aliens
+2 if the PCs control 75% or more regions over the aliens
-1 for each Weakness used during the mission
-2 if the mission ended in a TPK

Results:
3 or less: The aliens attack! They take over and control any one region that is adjacent to another alien-controlled region. If there is an available neutral region to capture, they take that one instead of a PC-controlled region. If they take over all regions on this world, the campaign has ended in failure and the PCs are lucky if they manage to get to the escape ships and flee the planet.

4-7: The status quo is more or less maintained. No one gains or loses any more ground.

8 or more: The aliens are forced to consolidate their strength, and abandon one region (GM picks). This region becomes neutral, and is not under control of any side.

4) Plan Next Move
Players decide which adjacent region they want to attack next. If they attack a neutral region, they automatically control it and proceed to Step 3. If they attack an alien-controlled region, play mission then proceed to Step 3.