The following picture depicts my problem:
I photographed some more:
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Pinch the two ends of the bag together and slide them on, I'm not sure if this works all the time but I used this method a few times and it worked for me.
First, get your finger underneath the tab, so that you are pinching the inner tab from the outside. Next, pull the tab from underneath as if you are opening a bag of chips. There should be a small opening now, from there, you should be able to easily separate the bag and re-attach the slider. :)