The best way is to have both gearcase and control in neutral. From my other post:
There is a molded plastic fitting that holds the upper shift rod in place, in the midsection. They often fall into the gearcase where they do not function, or they get lost. If you have this fitting, make sure it is installed in the midsection when installing gearcase. Stick it in with grease or something.
To install the gearcase: Tilt the motor all the way up. lock it up if necessary. Make sure the plastic water tube extension is in the waterpump output. Grease the splines on driveshaft and shiftshaft. Slide driveshaft into midsection. Insert copper water tube into plastic water tube extension. Push and hold gearcase into midsection. Turn flywheel until driveshaft splines mate. Push until shift shafts meet. They may mate or you may need to rock the throttle handle to help them mate. If you don't have the fitting, you may need to use a needle nose plier to move the shiftshaft, from outside the midsection, to help it mate.
Put a nut on the front stud and using only your hands (never wrenches), wiggle the gearcase on. Make sure the upper shift shaft is mated with the lower shift shaft, or it will be pushed up into the lower carb.
