After buying some cheap butterfly knifes, I had to try my luck at making some 3D printed trainer knifes. And here is what I've done. Of course, it doesn't have the weight of a real butterfly knife, however if you're a beginner and want a cheap and easy way to get gud, this is a nice option.
Print everything in 100% infill. This gives better rigidity and weight. You can also print the different parts using different colors to make a really cool looking trainer knife, as you can see I've done in the pictures.
Everything is put together by super gluing the pins in place. After you print the parts, first test fit to make sure they all fit together before gluing. Assemble it according to the assembly picture. The shorter pins go to the blade. Gluing the pins is very simple, you just have to be careful not to get glue on the blade or the handle lock. (You'll probably get some on your hand but it won't kill ya) Only put glue on the inside of the bigger rim of the pins where they rest on the handle. After that, let it set for a few minutes (may be longer depending on the type of glue you use) and you're ready to become a ninja!