This finder was designed to use as part of the library telescope program - http://librarytelescope.org. The red dot finder that is modified for use in this program is sometimes difficult to keep operational over during long-term use by library patrons. This is meant to be a replacement for it that does not require batteries or have any wires.
You will need 2 M3-.5x30mm screws along with hex nuts. The screws should have countersink-type heads.
You can print using any type of filament you feel comfortable with. PLA seems to work fine. I choose to print the two crosshair sights using glow-in-the-dark PLA filament.
If you are going to print the crosshairs in glow-in-the-dark or just a different color of filament, then print them separately. You can print the rest of the parts at the same time, although you may have trouble with the tall "outside.stl" part, which can possibly tip over if you aren't using a brim. I sometimes print that piece separately to use a brim so that I don't have to on the other parts. Up to you.
After printing, insert a hex nut in each of the pockets, front and back, of the "inside.stl" part. You will probably have a little struggle doing that, but they will fit in assuming you don't have serious shrinkage issues with your prints.
Next insert the rear loop into the slot in the inside bar. You can choose to glue that in, but for me, it's tight enough that I don't need to. You should insert it with the thicker part of the loop facing forward so that you will insert the cross-hair from the back.
Insert the crosshairs, minding the tiny tongue and groove to force the proper orientation. The rear crosshair is smaller than the front. I would suggest that the surface with the crosshairs should go in first. You will likely need to glue those in as well.
Insert the two screws. The forward screw should be inserted from the left as you look at the finder from the rear. Thread it into the knob. Both are identical. You will need to glue these in but you might want to test things first with no glue. Thread in the rear screw as well and into the knob.
That should do it.