The purpose of this Instructable is to show, step-by-step, a joke that can be played on a host of random people.
The chronology of the steps is important and each step has to be "sold".

Even more "hilarious" is involving your friends.

Best of luck this April 1st!

Step 1:

The first step is the planning phase.

You have to decide how great of a scope do you want the joke to encompass.
Should you go for a few of your friends?
or perhaps hitting a few unsuspecting random people would be more fun?
Also, you will have to decide how many people you want to dupe:
10s?
100s?
1000s?

Typically, this kind of joke will only be able reach a few hundred.
However, depending on your writing abilities (i.e. the ability to write fluidly and somewhat drawn out and nonsensically)
and your motivation, and I suppose your free time...
you might be able to get a few hundred more.
maybe even 1000.

i have attached my plan here to get a gist of the whole thing.
i apologize for my lack of dexterity.. but at least i can read it.
each step will be better explained throughout this Instructable (don't worry).

Step 2:

The beauty of Google is that it has a wonderful repository of outlandish (free) images.
All these images are readily available with the click of a mouse.

Try to find something somewhat outrageous but believable. An image where a moment of action has been captured, the result is obvious, and the consequences are left up to the imagination of the audience.
Try things like:
"Mascot Prank"
or
"Dwarf Prank"
or
"water prank"

Something simple but eye catching.

Step 3:

You may need to find some other random photos to really sell the authenticity.
Look for images of items that would be typical of a prank
(or at least used in the image of your prank)

Hoses
Balloons
Buckets
Feathers
Dwarves
Glass Bottles
Tacks
etc.

Step 4:

The great thing about sites like Facebook, Twitter, reddit, StumbleUpon, etc. is that they can reach a large number
of bored people looking for some sort of stimulation for entertainment or procrastination.

Depending on the number of people you want to dupe you will need to sign up for accounts on some or as many of these sites
as you are willing.
If you already have accounts for these sites then
SUCCESS!
Pulling off this joke became even easier!

A great thing about Google or Hotmail are the free email accounts they offer as well.

Step 5:

Bored people are naturally drawn to pictures, phrases, or promises of entertainment.
Things that are "cool" or "inventive" or "imaginative" typically get people's attention.

When thinking of a title to draw people in think of something that can pique interest.
You'll want something that people will read and then get ideas already flowing through their minds.

"Feel sorry for this guy; what an April Fool's prank!"

is better than

"Check this joke out. Ha ha."

Step 6:

Take your prank along with your awesome title and thought-tantalizing photos and start posting them to your
Facebook, Twitter, and other social accounts. Make sure the title (and picture) are prominent to attract maximum interest.

You can also forward the site of your prank to friends (or everyone) on your email list. If you don't want to look like that guy that
sends all the random stuff to your friends then open one of your old forwards from your inbox and change the subject to your title, attach your photo, and add your link to the body text.
Voila! Now you have a hilarious forward that you are passing along to your friends.

Eventually, random stuff like this prank go through the pipes and start hitting random people. You hope.

Also a tip,
A lot of sites monitor traffic and highlight so-called "hot" topics. So if you can then cross-post/reference your sharing but always make sure everything comes back to your prank.

Step 7:

Now just sit back and realize the prank is on you because of the time you just wasted.

Please send these helpful tips to all those you care about.
Please vote for me in the Instructables April Fools Contest come April 3rd.

April Fools.