Simply make a hole in the ribbon with a tooth-pick about 10-15cm (4-6″) from the end of the ribbon where it is affixed to the spool. Fit the larger piece of the fastener through the hole and then snap the smaller piece into place. Ensure that the smoother side (without the “bump”) faces toward the centre of the spool as the ribbon winds. This allows it to pass over the ribbon guide on its way to tripping the reversal lever.
Finding good quality ribbons remains a hit-and-miss chore for many of us. Sometimes ribbons arrive on the correct spool but with eyelets absent. I hope that this helps fellow typewriter users improvise to get the best from their wonderful Olivetti typewriters.