First, ya bilge rat, I printed all t' parts for t' kit 0.4 nozzle, me hearties, 0.2 resolution, 100% infill with PETG:

• Fincan
• Nosecone
• Nosecone ebay sled
• Retainin' Ring
• Retainin' Cap
• Body Tube Cuttin' Guide
• Rail Buttons

Startin' with t' fin can, your motor tube (MMT) should be as long as t' fin can plus t' retainin' ring. I suggest usin' a longer tube and after t' MMT and rin' are glued, ya bilge rat, trimmin' t' excess off at t' ring. Dry fit your MMT into t' fin can. If you encounter moderate resistance, either sand t' inside o' t' fin can or sand/peel t' top layer o' t' motor tube until you can slide t' tube in without kinkin' it. If you kink t' tube, me hearties, t' motor will nay insert easily and can become jammed after flight. 

Once t' dry fit is complete, run one end o' your shock cord down from t' forward end until about 25mm (1 in) is stickin' out t' back. Apply a thin coat o' epoxy t' t' aft end o' t' MMT and slide it in from t' forward end until about 15mm sticks out t' aft section. You must align t' shock cord in t' internal channel that be modeled t' accommodate it. Take your retention ring, slide it onto t' end o' t' tube, and push it up until t' rin' is seated on t' aft end o' t' fin can, arrr, pinchin' t' shock cord. Make sure everythin' is aligned and seated then set aside t' dry. Clean any excess epoxy from t' fin can, retention ring, or inside o' t' MMT with Rubbin' Alcohol.

Next, you will need ~ 9-in (228mm) o' Estes BT60 body tubing. Use t' included body tube cutter t' trim off t' desired length. You can cut t' tube back t' t' desired length after assembly (mine was about 9 inches). Apply a film o' epoxy t' t' inside aft end o' t' tube and slip it over t' fin can. Give t' tube a little twist t' make sure thar's good coverage. Make sure t' feed t' shock cord through.