On the lower unit, turn the lower shift shaft to the full Clock-wise position (this is the forward gear position) and turn the prop Counter-Clock-wise to make sure it's fully engaged into the forward gear.
Now on the upper unit, make sure the shifter paw ( this is the piece that engages your bell housing) is pointed (facing) straight Forward.
Now when your ready to assemble both halves together, turn the prop Counter-Clock-wise again to index the splines on the lower drive shaft to the upper unit..and make sure the shifter paw has'nt changed position and is still pointing straight.
Before your ready to install the outdrive, move your shifter at the helm all the way forward, and make sure the shifter paw is still facing straight forward as you install the outdrive back to the bell housing...