One trick to make it easier to get the cord to fit into the pincers end... I used to try to push the ends up into the pincer, which is a real hassle. What I do instead is... you thread the two ends of the cord through the plastic skirt (you know the part, right? That looks upside down with the wide end up). Then you grasp the cord ends to make sure you've pulled them taut, and you straddle the two ends of the cord on either side of the pinchers, and aligned so that when the cord ends fit into the pincer, the ends of fit inside the pincer and don't stick out.
Then, slowly press the sides of the cord ends inward into the pincers. If the cord is thicker than the pincer, you won't be able to get it all the way in--that's OK.
Now with the cord ends snuggly straddling the pincer, carefully tug at the cord from the looped end. Don't pull too fast; just tug at it slowly and evenly with one hand, while with the other, you make sure the cord ends stay smooshed into the pincer. Depending on the thickness of the cord, you will feel some resistance as the cord ends start to get pulled into the skirt with the pincer. Just keep at it, and at some point, you may need to periodically push in any cord that tries to push itself out of the pincer. But you should be able to pull the cord through, and the skirt will automatically act as an aligner to keep the cord ends in place.
Let the pulling motion and the skirt rail the cord ends in place for you so you aren't having to fight it.
See if that works.

