Take one piece of snap-off blade and put it into upper part. Use some pliers to avoid an injury. There is a small mark near blade pocket which shows you where the tip of the blade should be. Additionally you can use super glue to fix it in position.
Put both screws into the holes.
Put ball pen springs into the holes.
Place the upper part on the main body and press it fully.
Tighten the screws. Screw head should be about 1,5 mm inside head holes. If you will tight them too much it will destroy plate at the upper part.
Release upper part and check if blade is not visible inside the filament hole. If yes loose the screws little bit.
I recommend to put some lubricant inside to keep the mechanism running smoothly.