Not really a guide, but I can push you to your SSH skills.
Open iPhone Browser. Go to your theme of choice (Illumine has one
)
Go to Bundles > com.apple.springboard. There are 18 files needed. BatteryBackground.png, BatteryBG_1 - BatteryBG_17.png, and BatteryFill.png.
Just check the existing one for the dimensions of each image. I don't exactly recall.
BatteryBG_1 - BatteryBG_17.png are the images of the battery from lowest to full.