- +20% Land force limit modifier
- +10% Cavalry combat ability
- +1 Yearly prestige
- +20% Fort defense
- +1 Diplomatic relation
- −10% Cavalry cost
- −10% Stability cost modifier
- +1 Yearly legitimacy
- +25% Provincial trade power modifier
- −1 National unrest
First, you have to become independent from Poland. To achieve that, ask their rivals to support your independence. Poland is usually rivaled by Hungary, the Teutonic Order, Bohemia or Denmark at the start of the game. The first three are your best bets, since Denmark is far away which often causes them to refuse due to a modifier. If Poland rivals Muscovy or Ottomans it might be best to restart, since they probably won't support your independence. After securing independence support from all possible countries, start improving relations with the Emperor (usually Austria). Build up to 10 troops with infantry and declare on Poland when you're ready.
During the war, you should let the AI do the fighting and focus on sieging Polish provinces. It is recommended that you siege down Dobrzyn, Inoworclaw and Naklo before the Teutons. Peace out all Polish allies except for Lithuania since they will immediately rejoin the war if the Polish get the union over them (you can safely peace them out if Poland appointed a local noble). When you peace out Poland ask for independence and the three aforementioned provinces, don't worry about connecting your lands just yet. If you couldn't secure them before the AI, you could look for another way to border the HRE through Bohemia, but the game is going to be a lot harder since you won't have easy expansion routes. Core everything and when your relation with the Emperor is high enough, start adding your provinces to the HRE to join it. Once you're in, the game becomes a standard HRE game with an added benefit of easy expansion routes in the east, outside the empire. Finish off Poland by taking Polish culture provinces for yourself and giving others away to your allies or releasing vassals. Once Poland is gone, you could take the decision to form it yourself, thus achieving "Back to the Piast". From then on, Europe is your oyster.