i am assuming that you are replacing the hdd with a laptop hdd and need to format it for your xbox to use. if i am correct try this guide..it is for the older xbox 360 but i think it should still work
The old XBox HD Hack will work for the slim, I have tried it and it worked.
You will need to make some sort of suspension to hold the drive in the center of the slot, as you will have a bare drive in a bay that is expecting a Microsoft piece of equipment.