The way I normally do it is the following. Strip the sleeve further back than you need (say, an inch). Untwist the 8 cores and separate them.

Arrange them in the right order (the extra length makes this easy) Flatten, pack them together, and pinch with your thumb and forefinger near the base.

Without letting go of the pinch, use your free hand to cut them to the correct length. Now that you have them flat between your fingers in the right order, it should be pretty straightforward to slide them into the connector.