Evidently by violence. Above is the illustration accompanying this announcement at the British radical Muslim site [1] Al-Muhajiroun (thanks to "[2] Allah"):

Islaamic conference How the Khilaafah will be restored & Man-made law destroyed ...80 years on from the destruction of the Islamic state - 3rd of March 1924... Wednesday 3rd of March 2004, From 7pm to 9pm, Central London. For venue, please phone on the day of conference: 07939 737 727. For media enquiries call: 07956 600 569

Al-Muhajiroun seems to think, however, that they have a picture of the White House -- that's how they labeled the picture above. With intelligence of that quality, they won't be restoring the caliphate anytime soon.