The first step in building the Little Dog is the motor mount.  Take the smaller tube and  the 2 rings.  Place one ring over each end.  Push the ring on the tube so that there is a little space between the ring and the end of the tube.  Put epoxy where the yellow arrows are.
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Be sure where you put the rings on the motor mount, that they won't be in the way of the slots on the body tube.  Where the yellow arrows are, the fin must go thru this and attach to the motor mount.  If the ring is in the way, it won't be able to go thru.  That would be a bad thing.
Now that the epoxy has cured (hardened), I put more epoxy on it and then shoved it up in the body tube (on the side where the fin slots are).  Be sure to push the motor mount all the way into the tube.  The picture only shows it partway, but after I took the picture, I pushed it up further.

Get ready to put in a fin.  As you can see, I've already put on a couple of fins.

I put a little bit of epoxy to the bottom of the fin, and then put the rear corner of the fin into the slot.  Then I slid the fin all the way back until the front end went into the slot.

Like this.  Then you can put just a tad of epoxy into the fillet (the place where the fin connects to the body tube.

You should have 5 disks.  One of those disks will have the letters NC.  That is for the nosecone.  For the other 4, there should be two bigger disks and two smaller disks.  Epoxy a smaller disk onto a bigger disk.  This will be used on the electronics bay.  I went ahead and put a bolt in mine to keep it exactly centered.