Bookbinding needle and waxed linen bookbinding thread

Feed one end of the needle through the eye of the needle

You'll have one short end (the leader) on one side of the needle eye and the remainder on the other side

Partially untwist the the thread on the leader side

When untwisted you'll be able to see the three strands which make up the thread

Feed the pointed end of the needle through the gap in the threads

It doesn't matter which gap

Carefully pull the needle all the way through