Print one of everything with the exception of the Spring catch - you will need two of them.

I suggest printing the bottom of the casing with support, though its not mandatory.

You will need a spring that fits between the two catches, I personally grabbed one from the local ACE hardware. You don't want the spring to be too hard to pull apart so go for an easier spring rather than power. The comb really doesn't need much to extend it as its light.

On my printer the amount of sanding needed to make everything slide nicely was basically zero. Your machine may vary.

I printed mine in PETG

NB: Ok this should go without saying but this is intended as a novelty. Don't go doing anything stupid like modifying it to have a real blade. OTF switchblades are illegal in several states. Additionally be aware that schools etc have pretty much a zero tolerance policy for anything even remotely looking like a weapon. Be smart.