Just did it after saw your message.
Strategy quite straightforward:
1)Make an alliance with Denmark(through marriage)
2) borrow money, improve relation with pope
3) wait until you become an advisor(meantime if possible try to kill liege(personally did not succeeded in it), give birth to babies and marry them of for alliances(they will be helpful later)(I got only two boy and girl)
4) while advisor get favor with strong vassal of your liege
5) buy couple of favors from his other vassal(I bought two)
6) create faction make others join it
7) attack when liege fighting another war
8)after winning a independence war
9)breake truce (ideally if hre have an antipope, if not ask pope to excommunicate emperor)
10) win a war, hopefully you get a lot of alliances(I got 2 england and Croatia)
Hopefully you will get enough opinion with pope and have right traits at the end
Tips:
1) only give away counties that are de jure part of your duchy(emeror often will give your vassals a duchy title otherwise)
2) if you will have ambition to get title, then you will most likely get a duchy Ferrara
3) try avoid sins
4) submit to all factions that ask to increase council power
5) do not spend gold unless it is necessary for a goal
6)try not to focus too much on enemy army if your goal is excommunication, because he will be probably replaced before end of a war if he is weak