I made my first whoopie sling a few days ago. It does take some practice. Thanks to all who post videos of this. I watched several, and more than one time.
The wire has to be just right. I tried different devices to pull through the tunnel. It took me some experimenting to find wire that worked, and to get the end formed so it would work. At that, my first whoopie sling had a few rough places on it where the braid got disarrayed. I am still looking for a good crochet hook to use as a fid.
Keep trying. Watch some more videos. Those guys have had lots of practice. It is only easy after you practice a while and find or make the right tools.