Step-by-step process:
1. First day, immediately declare war of reconquest on England. Lose it on purpose. Offer to end the vassalization of all seven of your vassals to start the game.
2. Immediately set about annexing all the vassals with reconquest CB. This shaves a good 60-65 years off of uniting central France compared to the typical diplomatic annexations.
3. Get in good with the Holy Roman Emperor (relations >100), and add provinces to the HRE via the "Join the HRE" provincial decisions. You need to add Ile-de-France specifically, but you can't add provinces unless an adjacent province is already in the HRE. Take as direct a route to Paris as possible; the Emperor can reject your attempts to join, and aside from usual AI jerkish shenanigans, will be more inclined to reject offers from states larger than itself. You should eventually get in.
4. Get elected Emperor. Now that you're in the Empire, you get a 200 point swing in electoral votes (-100 penalty for not being in empire -> +100 bonus for being a large state in the empire), and you should thus be able to win election easily.
5. Fight a bunch of wars with other big HRE powers (Burgundy, Milan, Austria, Bohemia) and force them to release HRE states; each one released gives you 10 imperial authority.
6. When you're not busy, release some vassals after adding their would-be-capitals to the empire; you'll get 10 IA for this too.
7. When you get the chance, add the other provinces too, because each province added to the HRE gives the emperor 1 IA.
8. Use money from fighting the wars in (5) to pay for gifts to a significant majority of the HRE states (I had all of Germany + the Lowlands at >100 relations). A small HRE state with >100 relations with the emperor is a surefire lock to pass any reform you try to get through.
9. Pass reforms one at a time as you get to 50 IA. Get relations with everybody high before passing the second-to-last reform; anyone who refuses the reform leaves the empire and triggers a crazy civil war that's either exciting or a severe headache depending on your point of view.
10. Once you pass the last reform, congrats, you are now the Holy Roman Empire, which is still neither holy, nor Roman, but at least it's finally an empire.