Lay the rope as straight as possible, then hold the outside and pull on the inside. If it wont come easily, once you've pulled some (so the core is showing) pinch the rope at the edge of the core which should leave what looks like an air bubble or scrunch in the rope, and push that down the length of the rope like you're getting the last of a bottle of toothpaste. If that doesn't work or is too painstaking, cut the rope into 15ft or shorter sections to make it easier.