You should really use a Part P registered electrician to wire this up but you can fix the switch and running the 3 core and earth cable in a piece of conduit.
Wiring a 2 way switch is easy enough if you follow the diagram. The existing switch and the new switch must be 2 way so you can strap the cable between them.
The existing switch wiring to the ceiling rose stays the same but you need to connect L1 on the existing switch to L1 on the new switch, then L2 to L2 and Common to common.
I can't past a diagram into here but there are plenty of diagrams on the web. One here is good.
http://www.diynot.com/wiki/_detail/elec ... ache=cache