By learning how lighting circuits are wired and working it out.
It's a very simple thing to do, and if you can't work it out then you don't know enough yet to be doing it - working with electricity is not something that you should do on the basis of following "connect this wire to that terminal" instructions without understanding the why.
I can recommend the diagrams in the For Reference and Wiki sections, and the Which? Book of Wiring and Lighting.
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