Brandenburg is my favorite nation to play focused solely on European conquest (Portugal for colonization and England for a mix of the two) and this is my Brandenburg/Prussia strategy. I've used it at least half dozen times with great success; it works on 1.11 if you're still playing that, and it works even better on 1.12 with Paradox adding a bunch of new provinces to the Baltic area strengthening Brandenburg's position. I haven't tested it on 1.13, but it should work fine with that too. The only caveat being that with the change of AI allies only joining one offensive war every 10 years, you'll have to juggle your allies a bit and not call them into every war. After your first few wars, allies go from "necessary" to "helpful" unless you're fighting France or the Ottomans, so this shouldn't be too big a deal.
If you want to be colonial power, then this guide isn't for you.
If you want to go tall rather than wide, then this guide isn't for you.
If you want to become emperor and do the HRE thing, then this guide isn't for you.
If you want to stick to historical Prussia, then this guide isn't for you.
If you want to focus on becoming Germany, then this guide isn't for you.
This is a guide on how to create a wide Baltic & Northern German Empire as Brandenburg/Prussia.
Check to make sure all of the following are true:
1. Saxe-Lauenburg has a green heart towards you
2. Denmark and Poland haven't rivaled each other, neither one way nor mutual
3. Austria has rivaled Bohemia
If any of these aren't the case, then restart. Rival the following: Mecklenburg, Bohemia, and the Teutonic Order. Switch your HRE vote from yourself to Austria; +25 same religion, +25 they voted for us, and same rival bonus for rivaling Bohemia should allow you to immediately royal marriage Austria and then ally them. Use your other diplomat to royal marriage and ally Denmark and Poland. Look at the OPMs to the west of you, particularly Saxe-Lauenburg, Lüneburg, Magdeburg, and Anhalt, and RM + ally any who have a green heart towards you. Saxe-Lauenburg is the vitally important one as that prevents the Hansa from connecting their territory and allows Denmark to march troops into your own territory to help in wars. Go over the relationship limit if you have to, you'll want to vassalize all the ones who are friendly towards you as soon as possible and allying them shields them from the aggressive expansion you're about to take.
This may take a couple of attempts to get right, but you should have RMs + alliances with Austria, Poland, Denmark, Saxe-Lauenburg, and one or two other OPMs to your west within the first 4 or 5 months of the game. Focus on getting Austria, Poland, and Denmark first before the -20 too many diplomatic relationships modifier kicks in.
Keep an eye on Poland, you want to have all your diplomacy setup and go to war BEFORE they get the PU over Lithuania. One of the conditions for the PU to fire is that Poland has to be at peace, so if you can get Poland into a war before they get the PU and keep them there for a few years until Poland gets it's own king, then the PU will never fire. Yes, this will make Poland a weaker ally in the short-term, but that's much better than having to deal with a combined Poland-Lithuania in a century or two. If Poland gets the PU before you finish your diplomacy and are ready for war, restart the game.
Take the mission to reconquer your core of Neumark currently owned by the Teutonic Order. The Teutonic Order should be allied to Mecklenburg, the Livonian Order, and Hungary. Declare war on TO for Neumark. Austria and Poland should definitely be willing to join you; Denmark may not be, but they're not essential. Let Austria and Poland deal with Hungary and TO while you siege Mecklenburg. Annex Mecklenburg once that's done and start coring immediately, congrats you now have a coast. Next, move on to knocking Hungary out of the war, just force them to cancel their alliance with TO and give you some money and war reps, you don't want to weaken them as that gives the Ottomans an easier time expanding into Europe. Now focus on the TO. They have a lot of forts, so this could take awhile.
Poland should have the mission "Conquer Western Prussia" which gives it claims on the four provinces of Danzig, Tuchel, Kulm, and Marienburg, you're not going to be giving Poland any land so try to be the one to occupy these provinces yourself so that you don't get the -100 didn't give us claims or cores in peace deal relations hit. If you can't, it's not a huge deal since +25 same religion, +25 RM, +50 alliance should allow you to keep your Polish ally regardless. The only TO province you absolutely must occupy yourself is Memel. If Poland gets that occupation instead, restart the game.
White peace the Livonian Order, don't take anything from them (not even money), you want the truce to be as short as possible. Take Neumark and Memel, humiliate, and take money and war reps from TO; take as much as you can without taking more land, you want this truce to be as long as possible. You already have a core in Neumark and you should be able to core Memel as soon as Mecklenburg's Wismar and Rostock finish coring.
Since you eliminated Mecklenburg, you'll need to pick a new rival. Pick the Livonian Order if possible, if not Pomerania is fine. Once Memel is cored, fabricate a claim on Goldingen, the Livonian province north of Memel. Spend the 5 year truce timer with the Livonian Order letting your manpower recover, paying off any loans, building trade ships, and vassalizing your OPM allies.
As soon as your truce with the Livonian Order is over, you'll want to be at war with them, but you don't necessarily need to declare on them directly. The LO should be allied with some combination of the Hansa, TO, Riga, Lithuania, and Novgorod. If their allies are weak, attack LP directly, if not attack one of their allies and drag them into the war. For example, if they're allied to both the Hansa and Lithuania and you don't want to fight Lithuania yet, then fabricate on the Hansa and attack them instead, LO will have to defend them. In fact, it might be better that way, so that you don't have to worry about grabbing those 4 TO provinces before Poland can get them again (although that should be easier this time, since you won't be busy with Mecklenburg). If TO is in the war, take Ortelsburg, humiliate, and money and war reps; remember nothing to Poland and long truce timers with TO.
Your primary goal in the war is to get Goldingen, Reval, and Narwa from the LO. Once the war is over, core them and start fabricating claims on Ingermanland and Neva.
By this time Denmark should have taken Ösel, so you should be able to march your troops from Berlin, through Poland (get military access), Ortelsburg, Memel, Goldingen, Ösel (get military access), Reval, and all the way to Narwa to deal with any rebel trouble.
Back to the West:
By this time the AE you took from annexing Mecklenburg should have burned off, so while your eastern AE has time to go down, your LO provinces are coring, and you're fabricating claims on Novgorod, it's time to do some more expanding in the HRE.
There are three options here, the best one will be based on how your game is unfolding:
1) attack and annex the OPMs which were hostile to you at the beginning of the game
2) attack Bohemia, take their two Saxon culture provinces and force them to release Silesia
3) take the Pomeranian succession mission and eat some of Pomerania
You'll want to do all of these eventually, so there's really no wrong answer, just watch your AE and avoid coalitions.
You should have been able to do all this quickly enough that Muscovy hasn't had time to deal with Novgorod yet. Novgorod usually doesn't have any allies; so use your claims on Ingermanland and Neva to declare war on them, Denmark should be happy to help you. Take Ingermanland, Neva, and Novgorod in the peace deal. This is extremely important. Without Ingermanland and Neva, Muscovy can't build a navy, and without Novgorod, Muscovy can't form Russia. Since Moscovy can't form Russia, they don't get the automatic claims in the Russia and Steppes regions, which makes them less aggressive and easier to handle. Also, Neva and Novgorod have trade modifiers giving you some nice trade power in the Novgorod trade node.
The rest is up to you:
Congrats, you're now setup to completely dominate the southern Baltic in short order. You'll have an increasingly good presence in the Lübeck, Baltic Sea, and Novgorod nodes for income, you have very good strong allies, and the two sides of your country are far enough apart that aggressive expansion from one end won't effect the other.
Your strategy from here on out will be to oscillate between fighting in the HRE and fighting in eastern Europe. Eat some of Lithuania, then go attack the Hansa; chip away at Muscovy, then annex half of Saxony; rinse and repeat, etc. etc. Just always keep an eye on your AE.