You'll see a white arrow on the inside of the lens, where the aperture feeler goes. Set the lens to 5.6 on the orange scale (color coded to what the mount has the identifying P/ES etc in), so the pin fits into the metal slot that moves in the lens... Let me say this a better way: when lens is at f/5.6 there is a little metal fork with the center opening lined up to where the white arrow points, on the lens. The protruding pin (with a pointer) will go into that.
Not very difficult. The locking mechanism will move easily when everything is set down right, and you'll have access to all the apertures. If not, try again.