Step-1
Here's the trick part:
Slip the assembled mainshaft into the case and let it slide over to the right side, w/bearing adapter hanging out the back side. Have the syncro collar centered (neutral position).
Put the 2nd./3rd. gear lever in 3rd. gear position (lever backwards).
Insert the 2nd./3rd. shift fork in the hole in the shift shaft. It will be well ahead of the mainshaft, leaving plenty or room to get it inserted.
Move the 2nd./3rd. gear lever to 2nd. gear position. It will end up looking like the photo, w/shift fork nicely lined up w/syncro collar.