I dont have any tips but this it what the manual says.
Insert shifter fork (1) into the slot of mainshaft 1st gear.
Insert shifter fork (2) into the slot of the countershaft 3rd gear.
Insert shifter fork (3) into the slot of the mainshaft 2nd gear.
Now slide the FORK SHAFT throught the transmission case hole, through the forks and into the hole on the other side.