There's two main ways - the quick and dirty (which I did), or the slightly more involved.
All I did was drill into the connecting faces of the front and back, and connect them with brass rod. Then the gravity section can fit over it - there's a hole in it which fits nicely.
Or, if you want it to spin more smoothly, do the above, but put a section of slightly larger brass tube in the gravity section, and run the brass rod through that.
Be careful when drilling the front and back; if you use a power drill, it thens to heat up very fast!