Here are some helpful diagrams that show how lighting operating on a one-way switch can be changed relatively easily for two-way switch operation.
Turn off the consumer unit and remove the appropriate circuit fuse / miniature circuit breaker first before working on any electrical circuit, once the cable is installed make the connections to the two-way switches as shown.
Fig 1 and fig 2 show the original switch connected with original (old colours, twin Red and earth) switch-wires and the additional two-way switch linked with cable in the new colours, which in the case of three core and earth are Brown, Black, Grey and an earth-wire which should be covered with Green and Yellow sleeving. This is because electrical cable
Whenever an electrical installation uses both colours of cable, a self-adhesive warning label should be stuck to the front of the consumer unit.
The label normally reads: CAUTION – This installation has wiring colours to two versions of BS7671. Great care should be taken before undertaking extension, alteration or repair that all conductors are correctly identified. These labels can be obtained from most electrical wholesalers for a few pounds as a cash sale.
Replace the original one-way switch for a two-way switch, connect the wires as shown ensuring you tighten up the terminal screws fully. Make sure to connect the earth wires even if the switches are plastic, so there is an earth present throughout the length of the new cable, this also ensures there is an earth to connect to should the switches be changed for steel or brass in the future.
I hope you have found this article useful. As always, be very careful when working with electricity.