If you have Cannon 500d you know already that it doesn't have a external mic jack. But you can add one within a couple hours.

XCFloresX photography does an awesome job of this mod on a youtube video. Watch his video first!

My instructions are just a specific case for my camera and the 3.5mm jack I chose to use. I have the Asian model not the Rebel.

Note:
Make sure you get all the screws out before you remove the back case. The back case comes off quite easy. My camera had a screw hiding below a sticker.

When you take the back cover off be careful not to damage the ribbon attached to the circuit board.

Tools:
Small Philips screwdriver - make sure it fits properly in the head, these little screw heads will strip out easily.
Small Flat screwdriver - to pry off the back cover.
Drill
Hot Knife or a small saw, something cut plastic
Soldering Iron
Knife
Small file or Dremel
3.5 or 2.5 2 pole audio jack

Step 1: Placing the Jack

As you can see in the picture, the jack body is a little too wide to fit in.So I used a pair of bolt cutters to cut the material in yellow out.

Step 2: Soldering the Jack

Be careful cutting the internal mic wires. Allow your self enough wire to work and stip the wires. They are very tough to strip!
And I also bent down the jack tabs to ensue the back case would fit back on.
The back cover has a metal shield inside it so I added a small piece of electrical tape to ensure the jack contacts didn't short out.

Step 3: Back Cover Material Removal.

The yellow shows where I had to remove material from the back cover.
I used a hot knife to remove the plastic and then a grinder to remove small amount of metal required.

NOTE: Try not to get any small bits of plastic or metal into the body of your camera. Nothing good can come from that.

Step 4: The Test Fit

It took me 3 tries to get the cover back on and fitted properly.

Hint: I had plugged a 3.5mm male into the jack to help hold it in place while trying to get the case closed.

When it is done the jack made a very snug fit.

Step 5: Drilling a Hole in the Side Cover

Drill a hole in the side cover to just fit around the jack.

Hint: to get the hole in the right place:

put a piece of tape in the cover where the hole should be.
then align the side cover along the x axis and draw a line.
then repeat the procedure for the y axis.
where the 2 lines cross is where the hole center should be.

This hole needs to be very exact because there is no room for movement. measure twice and cut once.