That's like asking how do you 'do' speed, or how do you 'do' temperature.
Without a problem or exercise to work on, the best I can tell you is:
Memorize these two formulas, and then use them:
1). The probability that something will turn out right is
(the total possible number of correct outcomes)
divided by
(the total possible number of all outcomes, correct or not) .
2). The probability of TWO different things BOTH turning out right is
(the probability of the first one turning out right)
times
(the probability of the second one turning out right) .