I had one a couple of years ago so I will try and remember what I had to do. I think I used it for a Tyrannofex, and I didn't use all the bits.
The fit was poor and it needed a lot of putty. I don't specifically remember about excess resin, however if it looks out of place and stops the part from fitting, it most likely is surplus. You should dry fit the parts with Blu
Tac or masking tape before you use any cement.
So you've got four main bits. There are two bits of carapace, a small bit for the head to go on, and a bit to go underneath the chest with the egg tubes dangling.
You take the two halves of the Carnifex torso and put them on either side of the main, three pipe carapace. The side half pipes are meant to match up, but will need some putty to help them along.
The spiky bit of front carapace goes on the front, and has two slots that fit around the foremost side pipes. (Not very well.)
The smallest part is where the head is going to be put on. It goes underneath the front carapace.
I didn't use the egg laying part so I don't know how that will fit. Hopefully if you dry fit everything else with bits of Blu
Tac, it will be fairly obvious how that part is meant to fit. Though it is fairly obvious that there is a recess for the tail to go in so that tells you something.
As I said at the beginning, you will need to help things along with modelling putty. Fortunately that is easy to do on Tyranids because they are so organic, and once painted you will not notice anything. Also I would use araldite epoxy to cement everything. You need a lot of strength.