Paint a thin layer of Wellseal or similar non-setting sealant onto the mating surfaces. You can also fit a paper gasket, though it's not absolutely necessary.

Fit the sleeve gear into the main bearing.

Fit the two halves together. This will require a bit of jiggling to align the various shafts; whatever you do, don't use force.

At this point, it's a good idea to check that the shafts rotate freely and the gear selector works .