There is a how-to here, it's for a 2005 Focus but you can reference it for locations to mount the ballasts.
You would need to get H4/9003 kit and it's just a matter of wiring up the ballast and replacing the stock 9003 bulbs with the HID 9003 bulbs.
Here are some links to Amazon
where you can purchase the kits.
How-To Replace Bulb 2000-2004 Focus:
How to replace 2004 headlight bulbs: An adventure!
Bulb Size Reference:
Ford Focus: Bulb Sizes