Wiring a Light Switch

[powerman]

I need to add a light switch (A) which controls a new light fixture outdoors. The nearest power source for (A) is another light switch (B) which controls a ceiling light. The power comes into the ceiling fixture and then travels to (B). How do I wire (A) from (B) so both work properly?

[markmark]

This is something that should be done by an electrician. There are too many problems that can happen, e.g. fire, electrocution, etc.