Form 普鲁士 as soon as possible. Prussia is an ideal candidate for forming Germany because of their militant ideas and government. See the 勃兰登堡 and 普鲁士 pages for strategy on starting. Note that 波兰 will most likely have taken Königsberg (41) from the 条顿骑士团 so it is a good idea to ally Poland's rivals to obtain the province.
Once Prussia is formed, the player should improve relations with 奥地利 to form an alliance with them. If not Austria, the player should ally 法兰西 or a strong non-Germanic country. For instance, 丹麦 (Or 瑞典 if they overthrow and are stronger than Denmark), 莫斯科, 匈牙利, or even 奥斯曼. The player should use his/her discretion to choose allies. Austria may be the most critical alliance because they are frequently Emperor and are in the best position [aside from Prussia] to unite the German States, meaning if they go to war, Brandenburg [Prussia] would want to join them and control as much enemy territory as possible to prevent Austrian coring. However, 波希米亚 also has a high chance of winning the electorate. If that is the case, an alliance with Austria is key to get Bohemian land.
With a strong alliance, the player should begin to eat up North Germany, specifically gaining territories that lead to the provinces necessary to form Germany. For instance, Prussia should work their way through the Saxony regions to get to Brunswick (57), Thüringen (63), and Hessen (81). Prussia should continue with direct lines to the significant provinces until all are under control. It will not look organized, but is an efficient method.
It is important for the player to pay constant attention to heir-less German states that can become possible vassals. This allows for acquiring land without worrying about aggressive expansion, but only if the subjugation is caused diplomatically.