Putting Jailbroken iPad 1 into DFU mode
Plug the iPad into your computer
Launch iTunes
(turn of your iPad)
Hold down the Power button and the Home button at the same time
Keep holding both of these buttons for 10 seconds
After 10 seconds pass, release the Power button but continue to hold the Home button for another 3-5 seconds
When in DFU mode, your iPad screen will stay completely black. If you see an Apple logo or otherwise you did not enter DFU mode
iTunes will notify you that it has detected a device in recovery mode