Enerjet by Aerotech, Astrobee D Build, Part 23, Engine Mount Positioning

The pre-cut fin slots are removed before the engine mount.

TIP: Don't attempt to push the slot pieces inside the tube. They'll get stuck on remaining fibers and hang up inside. Cut and lift them from the outside.

Carefully remove any remaining masking tape left from the inside of the tube edge. Lift carefully so you don't pull up any Kraft paper wraps.

The engine mount is slid in place.

Here you can see the upper centering ring a little low, below the top of the fin slot. It'll have to be slid up until the ring bottom is even with the upper end of the slot.

Looking inside from the top - 

Turn the engine mount until the fin lock tabs are in line with the slots.

The mount isn't glued in yet, for now it is locked in place by the fin tabs. The TTW fin tabs are glued to the engine mount. There is CA glue applied to the root edges of the fins before clicking them into place. (See next post.)