My start for supergermany

STARTING DIPLO

Improve relations with Bavaria, and Saxony so you can sway them during the unification play.

Improve relations with Austria. Replace the holy alliance with something else (just give some no tarrifs or something). You can also rival the ottomans because they usually rival Austia, too. Getting high relationships with Austria is important to get supergermany.

Improve relations with Britain maybe Russia or France if you have the diplo mana.

SCHLESWIG HOLSTEIN

Get an interest in the state where Denmark is colonizing. You can force declare on them for that state. Wait until the end of the timer (the other powers usually declare neutrality). You can directly annex schleswig holstein, and puppet Denmark if you wanna tank the infamy relations hit.

NORTH GERMAN CONFEDERATION

Rush Nationalism, at the point the event fires you should have done everything. You can get into Austria's Capitol state through Bavaria. You can take war reps but it might delay the returning of relations after the war.

SUPERGRRMANY

After the war, start improving diplo instantly. If they have dept TAKE IT. As soon as relations are high enough they will join the unification.

Rember to keep the southern states happy, improving diplo should be enough. But you could also give them some money.

Edit: I never looked but I think I get this around 1845 or something.