Clamshell Wire Spool Box

This is a clamshell box adaption to keep spools of wire for electronics. To be honest I don't get the remix, remodeling thing yet. I downloaded the 3mf file and imported into onshape. Added the stem for the spool and then exported BUT I really had a hard time to export the model with the added stem. I actually have no idea how to do it again. I am still learning, if anyone can explain this process to me I would be very grateful!