I've tried to explain this in reply to an earlier posting. (6th June 2002 included below). I can draw you a picture, but a)can't work out a way of posting it on to the forum and b) don't think it will be any easier to follow.
The circuit for a single switch will be N direct to the fitting, E direct to fiting and down the switching cable, live to the switch, returned live from the switch to the fitting.
Always switch the live, not the neutral. This way the fitting is not live when switched off.
For a two way circuit, use 3 Core+E between the switches.
Leave the original live in the "common" of the first switch. Disconnect the original return from the switch and connect it to the red of the link cable using a crimp or connector block. Connect the link cable blue to L1 and the yellow to L2. Connect all earths to the box.
At the second switch wire blue L1, yellow L2, & Red common. Earth the box.
Don't forget to take all of the usual precautions, disconnect the circuit whilst working on it, sleeve the earth wires, don't nick the insulation, metal clad cables buried in walls, drill joists through the centre not closer than 1/4 of the span to the support or 4/10 from the support not larger than 1/4 of the joist depth not closer than 3 diameters to the nearest hole, and don't drill through trimmers at all etc. etc.
