It's not hard if you are practiced at it. A simple example is from Texas Holdem. You are initially dealt 2 cards of a 52 card deck. The number of possible combinations is:
52 X 51/2 X 1 = 1,326
So let's break the equation down
52 X 51 = 2,652. There are 1326 duplicate permutations so the 2,652 is divided by 2 X 1 to get the true number of 1326 combinations.
When you are dealt a hand say like Ace of Clubs and King of Diamonds the hand can come in two different orders (permutations). You can catch the Ace first then the King second....or you can catch the King first then the Ace second. Those are the two permutations of the same combination.