Here's some pics, one I did this past year.
The mount complete

The Base Cedar Stump

The bottom deer off, with the stump showing the square stock as Matt stated. This will be the "male" stock

The bottom Deer, I drilled out the form and put the larger of the stock, the "female" end, in the form and bondoed it tight into the form. It goes from the bottom to the top of the form, so that it accepts the male end in the cedar stump and the male end in the top deer. Both male ends, the one in the stump and the one in the top deer, are cut so that they are halfway into the bottom deer.
The top of the bottom deer.

The top deer with the male end installed. It is also bondoed into the form, as is the stock into the stump.

You have to take your time, start at the bottom and work your way up, and take your time! The big thing to watch is the "tilt" of the deer when together, and also to make sure they are not leaning to either side. Good Luck!