Rupert87,
Here's what I understand of the process. You need an abrasive harder and flatter than the stone you wish to lap. Ideally you need a lapping plate that is really flat, something where the runout is measured in parts of thousandths of an inch. For the hard abrasive part wet or dry abrasive paper works well. For the flat part, a piece of plate glass works well. Wet the paper and glass, stick the two together, and rub the stone on the paper on the glass. Many recommend drawing grid work lines on the surface of the stone to be lapped with a pencil so you can see the progress of the lapping. Keep the surfaces wet to get rid of the material worn off the stone. When the grid lines disappear, you should be there. Now turn the stone up about 45 degrees and rub the sharp corners of the stone on the paper to chamfer those edges.
Norton sells a lapping stone for their line of waterstones and it may be easier to use than the paper and plate method. I believe also that Norton recommends grit sizes for the abrasive paper to be used on specific stones, and I think I know what they are, but memory being a fleeting asset, I'll refrain misquoting the manufacturer.
The whole theory here is to create as flat a surface as possible on the hone as it removes a variable from the process. Lapping can also remove any glazing that may have occured if razor or hone parts have clogged the micro recesses in the face of the hone.
Bruce