You need two x two way switches and some three core and earth cable.
Easy bit first both switches are joined together like to like so L1 on one switch to L1 on second switch and so on.
Originally the first switch would be connected to com and L1 but with two way switching we connect to L1 and L2 instead.
So you end up with two wires in the L1 and L2 and one wire in com on the original switch position.
See http://www.ericpalmer.fsnet.co.uk/Lighting.html for diagram on how it works.
you will find the connection diagram in project section but basically you need to run a 3 core&earth cable between switch positions & purchase
connect earth wires suitably sleeved green/yellow at both switches, even if using all plastic switches & back boxes earths must be maintained (into small connectors if no provision in boxes.)
At new switch position connect one wire to each switch terminal, brown sleeve/tape the grey and blue cores first. NOTE what colour core is in your Com terminal (the one at the end on it's own), the other 2 go into L1,L2 either way round.
At original sw. after turning off supply disconnect the 2 cores connected to old switch, on new switch connect the colour core you noted as com. into Com. put one new and one old wire into L1, and remaining 2 into L2.
All these should be taped/sleeved brown. J-D.