Before you can attach the saucer, you need to do one of two things, and this depends how you built the saucer.
The part circled sits inside the saucer, so you either cut into your saucer or cut it off. Since I reinforced the "floor" of my saucer with pretty thick cardboard I opted for the latter.
And she is done!
I have to say I really enjoyed this build, lots of greeble opportunity, nice clear parts and overall a good looking model (If I do say so myself )
I remember reading that the Ambassador class was supposed to show the transition from the TOS/TMP Enterprise to the D shape, and I think they did it pretty well, but could have made a bit more reference to the B. (unless you want to count the squared nacelle pylons?)