All good advice so far.

The shifter fork shaft will come out of the case, it is not retained, just a sliding fit into it's housing.

Assemble the gearbox, including shifter hardware outside the motor first, then slide into position.

Be careful when you poke the countershaft through the countershaft seal though as it's not too difficult to damaged the seal or pull the spring off the inside lip of the seal if you're a bit rough.