Gatter an army and start taking them out one by one. Your victory conditions include most of Germany, so that is the place to start.
Farms and roads serve a double purpose as they’ll also increase the replenishment rate of your troops. Also build up your land-based economy with farms, roads and factories. The TP’s to the north-west of Great-Britain are good ones to aim for as they usually won’t be attacked. Though you are not a nation famous for its navy, getting a few Merchantmen on Trade Posts will help you out, so train a few of them and send them out. Try to maintain good relations with them, so get a Trade Agreement and send them a gift if they start to dislike you.
You won’t need much of their lands for victory so alliances won’t get in your way for the time being. With possible enemies on all sides some diplomatic protection is very welcome as Prussia, so try to ally with Austria, Russia and Great-Britain (and Sweden if possible) before you do anything else. Below I’ll outline a strategy in which you join the Coalition so you can focus on uniting Germany. The game pushes you towards the Coalition, but with your large kingdom in Germany and Poland and a well-disciplined army to fight your foes there are multiple ways to victory.
Prussia is a continental power that can make or break the Coalition or France. This is a general strategy for Prussia I wrote yesterday.
