Math behind flush odds

[magithighs]

I'm trying to figure out the odds for the flush draw (2 flush cards on the board and two in hand) to remember the principles of the calculation.

It's been a long time since I studied probability, but I remember that when calculating an "and" situation you need to multiply the odds of the events, and when calculating an "or" situation on two mutually exclusive events you add the probabilities.

Are these mutually exclusive events -- the flush card on the turn "OR" the river. So do you calculate the odds on the turn, and then add the odds on the river?

I'd like to know the math behind this -- not necessarily the answer, so I can better calculate "true" odds of events happening.

Many thanks for your help.
Magithighs