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If you're playing as Karl Karling, the reqs are under the HRE event fire in the intrigue screen. I believe you need to control Bavaria, Saxony and Italy (obviously firing the event to inherit your brothers kingdom) and have X prestige/money. If the Carolingian Empire falls I'm not sure what the reqs are. I never let the Carolingian Empire fall if I'm playing.
 
If the Carolingian Empire falls, the de jure territory for the HRE appears and I think you create it from that. Never done it myself, but it always seems to be whoever controls East Francia and Italy who does it, so I'm guessing it is.
 
If the Carolingian Empire fall, you need to hold the title of King of East Francia/Germany AND control at least 6 de jure duchies in that realm. Then of course Pope needs to agree.