This model represents a planetary gear with a transmission ratio of i=6.
I tried to keep the model easy, so the amount of extra parts and additional work should be minimal.

To get the tolerances right, you may have to experiment a little bit with the size of the gears,until everything runs smoothly.
For me, this scaling worked the best(machine: Creality Cr10):

Planet Carrier: 97%

Sun Gear (The smaller Spur Gear): 90%

Planet Gears (The bigger Spur Gears): 96%

Assembly: Like every simple planetary gear. You can bolt everything (especially the extra parts) with M5 screws and spacers together or towards something. The default axle cutouts are for hexagon axles.
But you can also melt any other shape that fits into the Sun Gear or Carrier. If you want, you can try to stack multiple of these gears in a row to get a ridicilous gear ratio. I haven't tried
that yet and I doubt that the plastic parts will withstand higher torque or rpm. Maybe if you submerge everything in silicon oil...
The extra parts turn the model into a functional LEGO compatible motor+gear (please use low voltages/rpm if you want the Part to survive longer...)
The motor I used was a "Tamiya Dirt-tuned Motor (27T)" (no ad, this motor just happened to lay around...) But you can use whatever motor you like, you may have to adjust the mount a little bit.
Also, you might need an adapter for the axle of the motor(usually really easy to print if you don't have a metal one).

That's it for now, I guess, hope you have a lot of fun building, playing and messing around with my model :)

PS: This is my first upload of a 3D model and I am not very good in English...