Step 3: Head: Measure it - Stick it
Now get the ruler and use it to find the vertical and horisontal middles of the facing side of the clock box, it might help to put little lines through them with a pencil. Now try to fit the clock box to the back of the brake disc and make it level. By level i mean that you can try and arrange the disc top and bottom (depending on how the large holes are placed throughout the disc) so that you have different holes for every different hour marks of the clock. I had to do this as my finished clock will not have any numbers stuck on it, so i would still be able to tell the time! See picture, this might explain it better. For the glueing process, you would align your 1200 with 1800 with the vertical centre pencil line on the clock body, and the 1500 with the 2100 with the horizontal centre line on the clock body.
Now, once you found the way one will fit to the other, get your superglue, make sure all surfaces are dry and free of dust, and start applying the glue, minding the surface of the disc to avoid the holes (see picture notes again). Squish the two together (be careful to do it properly - this is where the pencil markings of the holes come in handy), wait a minute - and its ready!