Just unscrew the top of the post, enough for the string to fit through, and line it up so the string can go through in a straight line.
Then feed the string through and pull it fairly tight with your left hand. Keep pulling the string in a straight line and start turning the tuner key with your other hand.
Nothing will happen for the first few turns, but eventually it will "bite" and you'll see the string start to wrap around the post.
At this point, I carry on pulling the string clockwise as the tuner is turning anti-clockwise, to let it tighten a bit more, then let go and tune up. Some people tighten the post a bit with a screwdriver or coin, but I don't find it necessary.
(Edit: And Wez has beaten me to it. :lol: )