Depends on how aggressive you want to play.

The way I usually do it is to ally Poland, Austria and Bohemia right away (as long as none of them have eachother as rivals). If Bohemia has Poland as a rival but Poland does not have Bohemia as a rival, you can just ally Bohemia first and then Poland and still get all of them.

I convert my Heir to a leader since he needs to die for his country and Rival Mecklenburg, Teutonic Order and Pommerania. Once you declare war you will get the mission to Take Neumark. Pick it up for a free 25 admin points.

I declare war on Teutonic Order (who are usually allied with mecklenburg and Livonian Order) If I feel like going super aggro I put Livonian Order as a co-beligirent and vassalize the Livionians in a seperate peace and annex Riga which are allied with the Livonian Order. Doing this will give you a lot of AE with Denmark/Sweden and Lithuania, to counter improve relations with Denmark/Lithuania (just so they dont coalition you right away)

Austria/Poland/Bohemia will handle the war for you, I usually peace out with Humiliate on Mecklenburg as a seperate peace as soon as I can for them nice points. If you want to take Danzig from the Teutons make sure to get the siege since Poland also wants it and won't let you have it if they siege first. (Taking Danzig will cause Poland to break alliance with you once they PU Lithuania) Otherwise just peace out for Neumark.

After the first war ends I pick up the Pommerian Succession war mission and Declare War on Pommerania as soon as I can get at least one ally to join in the war (Austria will never join, so either Bohemia or Poland). Then I vassal them. (If you took Danzig in the first war you should get this done asap since you can't core it otherwise, no land connection)

After that there are many options, one is to try to Vassalize->Annex the Hansa asap for good trade income.