There is a how-to here, it's for a 2005 Focus but you can reference it for locations to mount the ballasts.
http://www.focusfanatics.com/howtos/hid_install/
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 and
eBay where you can purchase the kits.
How-To Replace Bulb 2000-2004 Focus:
http://www.focusfanatics.com/forum/showthread.php?t=271229
Bulb Size Reference:
http://www.focusfanatics.com/forum/s...ad.php?t=34666