De-jure Empires require you to control 80% of their territory, have at least two Kingdom titles or already be Emperor and at least 600 wealth and 400 piety. Some have a few special conditions: Francia for example requires you to have a Kingdom title that isn't France, Aquitaine, Britanny or Burgundy. When I say it's the easiest way to become Emperor with Charlemagne, it's because you fulfill most of the conditions the moment you inherit Middle Francia from Karloman: this gives you all of the de-jure Empire's territory, two Kingdoms and one of them is Middle Francia/Lotharingia/Austrasia... By that point, all you have to do is basically amass wealth and piety and then boom you're Emperor. The wealth isn't a problem because with all you get, Charlemagne generally gets a pretty sweet income at the start: you might only be slowed down by the need to created Duchies in order to do vassal management. Piety is probably trickier to get but Charlemagne starts with the Zealous trait, which gives you +1 piety per month, and large realms generally have less trouble amassing piety. Of course, to make this easier, you probably shouldn't create Extra Kingdom titles (which isn't necessarilly a good idea early on anyway since you start on Gavelkind succession).
In the 769 start, the HRE is only formable by decision since it technically doesn't exist. However, to make said decision available, certain conditions have to be met:
- You need to be Catholic or Fraticelli. Charlemagne starts as Catholic so that's easily fulfilled.
- You must not be the Byzantine Emperor. Charlemagne isn't an unless you are very lucky, he likely never will. Plus, if you're already the Byzantine Emperor, you don't really need the HRE...
- Your ruler must be an adult, not in prison and not incapable. Unless you're unlucky that's not a problem.
- You need 1000 prestige. Charlemagne gets there pretty easily and pretty quickly thanks to what he ends up owning.
- It costs 2 years of wealth. Not that much of a problem given your revenues. Might take some time though.
- You need to own one Kingdom that belongs to de-jure Francia, Germania, Brittania or Hispania. You're King of West Francia/France from the start so done.
- There mustn't be a Christian Emperor of Francia, Germania, Britannia or Hispania in existence. Unless it's you. The likelihood of the AI doing that before you get your chance are pretty low, especially in Hispania given the Umayads.
- You must be King of Italy or Emperor of Italia. If you do the Lombard princess event, you get a claim on Italy and the conquest isn't that hard... Though prepare to suffer from being way over your vassal limit following the conquest. That's one of the complications.
- You need to have your Religious Head have +100 opinion of you. THAT'S A NIGHTMARE OF A CONDITION TO FULFILL. You need to acquire a lot of positive traits and no traits opposite to the Pope's: that requires a lot of checking to do. You can also bribe the Pope to raise his opinion of you but that generally costs quite a fortune, unless it's right after the Papal conclave... Other monetary problem comes if you choose to support one of your cardinals so that he becomes Pope because it costs money and requries careful management... Last possibility is to crown an Antipope then attack Rome to make your Antipope the Pope: it's extremly risky. But the real kicker is that this will also be extremly dependant on luck and events as Popes are mortal men and can die... So if you don't have fulfilled all the other conditions before making sure of that one, you're in trouble.
In other words, the conquest of Italy and the Pope make that option a nightmare when you play as Charlemagne. You're better off trying your luck at creating the HRE with one of his successors rather than with old Charlie himself.
The last solution is to create a Custom Empire, if you have enabled the rule of course (the other condition is to have the Charlemagne DLC which you obviously do in this case since you can't play Charlemagne without said DLC...). Custom Empires requires you to have 8,000 prestige and a realm size of at least 180: it's pretty easy to do that with Charlemagne given how large his Empire becomes. Alternatively, you need to own three Kingdoms: that's easily done since you can inherit Middle Francia early and then probably will go conquer Saxony or Italy... That being said, it's probably way easier and more convenient to go for the existing de-jure Empires by comparison.
