3 way light switch using a two wire control
I’m including this method for reference in case you find it used in your house wiring but would not recommend this approach in a domestic environment. If you are adding wiring for a three way light switch then use the 3 wire control system.
I have detailed, quite extensively, why a two wire control system can be problematic in the article 2 way switch with 2 wire control. All the issues raised there apply here too, especially if used in stairways and the switches are on different floors.
This method is best suited to conduit/trunking based installations where single core conductors are used.