I used my craft knife to slice horizontally through the layers of the chipboard.
Once I had made a start with the knife I carefully peeled off the top few layers.
Once I had separate the two layers I was able to cut the border off.
This is what I was left with after I had cut it out.
There is a wealth of tips and techniques over on the forum from all of the design team, definitely worth a look and there are new tips each month.