You'll start out with the usual aspects of Prussia. To the south of you is Austria and her allies/spheres, Bavaria, Wittenberg and Baden. To the east is the Russia Empire, to the west of you there is Belgium, the Netherlands and France, and to the north of you is Denmark with Holstein, her puppet, and Sweden. Now, the 1st step you could do is to try to sphere Hannover. Hannover starts on the Sphere of the UK, but since the brits are focused more on other sphering issues, it can be fairly easy to knock off Hannover from the sphere of influence of the UK. After the UK gets the "Victoria's coronation" event, Hannover will no longer be in the UKs Sphere, so you can sphere them easily from there. After that, you'll have to fight Denmark over Holstein. You will already have a free Clauses bellli on Denmark for "liberation".(ONLY FOR AHD OR BELOW). In HoD, you simply justify war to get Danish Schleswig-Holstein. While in the war, make sure you add "acquire Schleswig-Holstein" if you can defeat the Danish forces as that will be a key factor for forming the North German Federation, a key nation before forming the German Empire. An alternate rout is to simply sphere Denmark. You'll still be able to form the North German Federation, but just without Schleswig-Holstein.
Now after you've taken Schleswig-Holstein from Denmark, sphered Hannover and also added Holstein to your SoI, the last step till forming the North German Federation is sphering Saxony. Saxony is in the SoI of Austria, so it will be quite tough to un-sphere them. If needed, you could use war to take Saxony from the SoI of Austria, but I suggest using the diplomatic approach since it's less costly and easier than fighting a large war.

After you've done that, you can enact the "Form the North German Confederation" decision which will form the North German confederation.

Well done! You are now the North German Confederation! You now own all those small minor German states of the North, so you can build up more troops now than before! You'll need to do so, since the last step will require war. Ensure that you have good allies (especially the United Kingdom) so you have friends to fight against France, since they are the last thing standing in your way to the German Empire.
A possible option you can attempt to do, if you want, is to pick on Austria to make your empire bigger in the future. You don't have to do it, but if you want to, go for it.

The last step you need to do is to declare war unto France to liberate the territories of Alsace-Lorraine. There is already a free clause=belie for that for you, but the hard part is winning the war. Usually, the easiest way to end the war is if you or one of your allies have occupied Paris. Then, a decision will appear that will make France secede the territories and the war will have ended. After that, you can form the German empire!

Once you form the German empire, the southern German states will now be a part of your country! You can now feel free to dominate your enemy Austria or take on other nations if you wish around you.

Credit for the video and method described below belongs to shenryyr2. Be sure to check out his channel for more game stuff.
At the start of the campaign Austria will be interested allying herself with someone, so you might as well be that someone. Austria makes a great ally during the war with France over Alsace-Lorraine. France has a ton of troops and good defenses on your border, so letting them invade you a bit helps considerably- they will weaken in your lands, and you can attack them as they spread out and suffer attrition. Each time you beat them in a battle you get bonus to warscore. This tactic also works well against Austria.

Schleswig-Holstein is up for grabs after it is freed from Denmark, so I would suggest letting Austria do that war (they will if you don't) so you can sphere Schleswig-Holstein while Austria cannot influence them during their 5 year truce. If you go to war with Denmark, you'll be the one who can't influence Schleswig-Holstein for 5 years. In that case, you'll have to hope Austria gets it sphered, or wage influence-wars with whoever does sphere them.

Before declaring war on Austria, make sure all of the minor German states have been sphered by you or by Austria. France sometimes meddles with Baden, and at the start of the game Great Britain has Hanover sphered. Bavaria might become a great power- if that happens you'll need to do a Hegemony war against them as well which should be simple.

When all of them are either in your sphere or Austria's sphere, you're almost ready. Next, go through each of the ones in Austria's sphere, and Decrease Relations a few times. This will make sure they join the war, on Austria's side against you.

Now, declare Hegemony war on Austria, but do NOT call any of your allies. It is important your sphered German states are not threatened in any way, so don't let them join the war. Fighting the war itself is up to you, but ultimately you want to have Austria completely conquered, 100% occupied by you. Do not get peace.

Sit around and watch what happens to Austria- Firstly their Military ranking will drop to zero as you kill all her armies and navies. Secondly their Industry ranking will drop to zero as all factories in occupied territory shut down. Lastly, their Prestige drops to zero as they are forced to declare bankruptcy and all nations who gave them loans gain Repay Debts casus belli.

Eventually, Austria will drop from Great Power status. This is your time to celebrate, because all those states that were in Austria's sphere are now free to take. Go to each of them and add wargoal Add To Sphere, then peace them out individually. Next go to Austria, and do the same thing. They will be in your sphere now, and all you need to do is go to Politics window then Decisions tab, and Super Germany is yours.

An alternete way to form the Greater Germany is to form the NGF 1st and then declaring war on Austria later. Since there ain't no more mini nations, it can be truely up to you, with all that man-power

If you have any tips or anything you want to add to the guide, go to this link to add in your ideas for this guide, so that you can add to this, for I'm sure many of you people have better ways to form Germany. Thank you for reading, and I hope you enjoyed this guide.