I cut the ends off (use a hobby/craft saw or have even used a kitchen bread knife-the one with the jagged blade)
then use the 'center' part of the plastic spool as an insert into the center hole of the larger spools. This 'insert' fills up the hole in the larger spool and centers it on the spool pin of the sewing machine. After the end pieces are cut off, if there are rough edges on the ends, these can be smoothed down using a fingernail file.