See this video for a quick demo, as well as a quick explanation. If you want further reading, check out the rest of this instructable.
Step 1: Set Up Your Packet to Count
It goes without saying, but I am going to say it anyways, the spread cards are an "exposed" view, and the spectators never see them like this. You will use the Elmsley count to count the five cards as four, and also to hide the 3rd and 4th cards from view.
Step 2: Count the First Card, the Ace of Diamonds.
Step 3: Dirty Work Time
The ace of diamonds goes under the whole packet, and gets re-gripped by the right fingers.
SIMULTANEOUSLY, (or maybe a nano-second BEFORE) the right fingers push off all the cards of the packet except for the bottom one (soon to be second from bottom, after you add the A. of D.)
The left hand then carries away this whole packet of pushed-off cards, after having ditched the A. of D. under the now face-up A of Spades.
Step 4: Fairly Count the Last Two Cards.
Step 5: This Is Not a Step, Only a Few Comments.
To many magicians, especially this new younger generation weaned on MTV-style MagicPorn with blazing music and card juggling, the best way to hold the cards in the elmsley count is a topic of great debate.
I know of no reason why, though. To me, I have done the elmsley with both grips, and both work fine. Try them both and use whichever one you like best. And please, don't flame me with hate mail hating on my technique. It may not be perfect, but it has been fooling people since Christ was a corporal, so keep your comments to yourself. I am just showing the way I do it. I know there are other ways to do it, and if yours works better, use it. Just don't email it to me asking me to try it. I like these two methods just fine and I haven't seen any better ones.
Oh, also, this is called the Elmsley Count because a very fine English magician named "Alex Elmsley" invented it. Props to you good sir, wherever you are!