There is no standard way to mount a MiniCIM to a versaplanetary. However, Spectrum 3847 has an excellent manual on how to do this, found
here on their blog.
As for having everything you need, the basekit plus the two (3:1 and 4:1) stages should be all you need for a 12:1 ratio. For mounting, there are some threaded holes on the gearbox itself that can be used to screw it onto whatever you are mounting to. We have been very satisfied with the versaplanetary gearboxes so far.
EDIT: I just realised that the screws for the gearbox and the MiniCIM are a little bit different, and is mentioned in the comments on the blog post. We use a different bottom for the gearbox that we machined which has the correct threads to mount to the motor. There may be other ways to get around this - I'm not sure.