Distinguish between the inner and outer side of each spool half. The inner side is smooth while the outer side has grooves and a cutout for the label.

Align and insert each of the 3 short pieces with the corresponding slots on the inner side of the spool. They may be a bit tight.

Place the filament re-fill into the partialy assembled spool.

Align the other half of the spool and press to complete the assembly. Now you can cut the straps with your utility kife and pull them out. Done!