iPad Mini Checklist,
POWER OFF that device
No metal tools
Remove the long pads on the bottom of the LCD
Move tape from old screen to new screen (near the home button flex, or use a non conductive tape to cover all the exposed circuits)
If its pre-assembled and the power button is not working, its either a bad solder or bad IC Chip (you can try retouching the solder, but if its under warranty I wouldn't mess with it unless its a last resort)
Mini's give me a different problem everytime.