First, I printed all the parts for the kit 0.4 nozzle, 0.2 resolution, 100% infill with PETG:

• Fincan
• Nosecone
• Nosecone ebay sled
• Retaining Ring
• Retaining Cap
• Body Tube Cutting Guide
• Rail Buttons

Starting with the fin can, your motor tube (MMT) should be as long as the fin can plus the retaining ring. I suggest using a longer tube and after the MMT and ring are glued, trimming the excess off at the ring. Dry fit your MMT into the fin can. If you encounter moderate resistance, either sand the inside of the fin can or sand/peel the top layer of the motor tube until you can slide the tube in without kinking it. If you kink the tube, the motor will not insert easily and can become jammed after flight. 

Once the dry fit is complete, run one end of your shock cord down from the forward end until about 25mm (1 in) is sticking out the back. Apply a thin coat of epoxy to the aft end of the MMT and slide it in from the forward end until about 15mm sticks out the aft section. You must align the shock cord in the internal channel that was modeled to accommodate it. Take your retention ring, slide it onto the end of the tube, and push it up until the ring is seated on the aft end of the fin can, pinching the shock cord. Make sure everything is aligned and seated then set aside to dry. Clean any excess epoxy from the fin can, retention ring, or inside of the MMT with Rubbing Alcohol.

Next, you will need ~ 9-in (228mm) of Estes BT60 body tubing. Use the included body tube cutter to trim off the desired length. You can cut the tube back to the desired length after assembly (mine was about 9 inches). Apply a film of epoxy to the inside aft end of the tube and slip it over the fin can. Give the tube a little twist to make sure there's good coverage. Make sure to feed the shock cord through.