Normally the outer stem shoud be tapered in width over the last 10" (approximate) from full width down to 3/8". A mortice should be cut so the outer stem slips in place. The mortice should be cut down to the surface of the inner stem and tapered also. The outer stem should be 1" shorter than the inner stem at the inboard end to prevent opening up a hole in the hull when you cut the mortice.

I have some pics and info on this process on my MSN photo site at the posted link.

Glen in Baie-St-Paul.