4. Choose a film size that is large enough to surround the extents of your Design. Note: MAke sure to choose a 1 x 1, so it will be the only layer per film. You can merge multiple gerber layers onto one film, layer but I believe for this tutorial purpose it's easier to understand 1UP -- and this may be what you're after anyways.
5. Select here if you wish any layers to be Negative (Inverted) or Mirrored.
Notice that FAB 3000 has created a separate layer for each Gerber (i.e. film). You may now export those film layers as Gerber.