To start, take "A" and "B". Slide A into the top slot of B like this:
Set it aside.
Next, slide "C" into the bottom slot of "D" like this:
Now, take the first set of A&B and slide it into the top slot of C&D like this:
We're going to put the rings on now. (both rings are the same on the pumpkin)
Slightly bend your inner pumpkin (A,B,C,&D) to fit it in the center of the first ring and slide the ring down to the first slot:
Also align the inner pumpkin so that it's edges fit in slots directly across from each other:
Do the same thing for the bottom ring.
This next part is a little trickier. You're going to spread the inner pieces around the circle to the other slots.
Looking at the pumpkin from the end of it kind of like the picture above, move the top two inner pieces on the left 90 degrees to the right.