The motor mount is built, and the fins are attached. (Note: sometimes this part of the rocket is also called a “fin can.”) What next, you ask?
Well, next, the motor mount or fin can goes inside the rocket body, and it’s glued in place. Because the fins go “through the wall” and are already solidly attached directly to the motor mount inside, this method requires cutting the rocket tube slightly in order to slide it over the fins. The cut tube can always be sealed up again later with wood glue.
If you look closely at this first image, you can see where the rocket body tube was cut for each fin to slide past it. Fits like a glove!
As a side note, this end of the rocket is the “aft” end. Aft means rear. I confess that I did not initially know this. You may already be more acquainted with nautical terminology than I am.
You can’t see them here, but remember the motor mount tube has three wooden centering rings. Just before each centering ring slides inside the rocket body, flush against the body tube wall, you can add some wood glue to seal it. You can also add lots of additional glue to the final centering ring at the aft end, but that can easily be done anytime after this.
Finally, while the fins are already attached on the inside to the motor mount tube, they should also be glued again on the exterior to ensure an extremely secure bond.
You know what they say about a rocket that loses a fin.
Actually, I don’t know what they say, but without getting too deep into aerodynamics here, if your rocket loses a fin it will definitely be unstable during flight and will crash. And then you will feel bad.