Ambition: King of Frisia
I, as a dutch, love to start with Holland and create the Kingdom of Frisia. So I did. :)
First I build up my economy (gold win wars, not men) by building castle villages and towns in the two starting provinces. Also let your steward collect taxes in your capital.
Next, wait for an opportune moment to seize Sticht and Zeeland (your 2 starting de jure claims because you are the Duke of Holland). Those moments take some patience. Sticht can be hard because the Duke of Köln or Lower Loirraine can be quite challeging. However your economic base should give you enough means to pay 75 gold for some mercenaries who can do most of the work. Waiting till the rivalling Duke is allready in a war of just finished one, helps a lot! Same goes for Zeeland, which belongs to the Duke of Flanders. The Duke of Flanders has the tendency to declare independence from France, which could be a good moment for you to invade. Make sure if you use that timing, you avoid the armies of both Flanders and France, cause both are your enemies. Let them finish eachother off and get as much territory as fast as you can (before France does). If you are quick and well prepared, you will get a peace off from the Duke of Flanders way before the King of France wins his war (with which he would have ended your war too, if his war finished quicker!! So be aware of that).
After this, keep building more castle villages in your keeps and a strong levy in Holland. If you claim the barony in Utrecht, you can give the bishopric of Sticht back and keep the barony which will become the main fief in Utrecht then. You will want to get over 11 gold per province from each of your own fiefs and making sure you have high relations with your vassal mayors and bishops helps a lot with gaining gold too! With increasing your levy I focus on maxing out the archery range first at level 3, which will give you lots of numbers in your army (I seem to feel combat is mostly about numbers).
Then get claims on the Belgium (Duke of Flanders) provinces. I went for creating false documents (get a good diplomat and reserve about 150-200 gold per Count claim or double for a Duke claim (go for that if your diplomat is good enough 15+, preferable 20+)) on all his provinces (make sure to press them by war before your character dies, as those claims will die with him). Get as much claims on his as you dare before waging war (and dieing) so you can press all your claims in the same war (and don't have to wait out the 10 years of truce). A good moment (which I used) to seize them is in one of the Flanders wars for independence. Again, just make sure you are faster then France, so keep an eye on the war declerations between them so you don't miss out. It becames easier when he wins his independance, but again, be carefull not too wait too long, since the King of France is very eager to get it back under his influence.
When you have both Duke titles (you need at least 2 Duke titles to create the Kingdom title) and you have enough provinces, you can create the Kingdom of Frisia. If you get enough Flanders provinces, you can usurp or create the Duke of Flanders title. If you can create it (but lack the cash), make sure you do it fast, before the Emperor does it (yes, he can do it too). If you lack provinces to get to 50% of the provinces needed to create the Kingdom title, get more from the Duke of Brabant or Friesland, whichever is the weakest. Just get a claim again, and declare war on the Duke.
Overall you have to be wary of the Medium Crown Law, since that limits your ways to wages internal wars in the HRE (with for example the Duke of Brabant, Friesland, Köln or Lower Loirraine). So making your moves fast is essential.
This is only my second game with Crusader Kings II, so there might be a faster with with getting your claims and territory. I only got Count claims, but if you are able to get the Duke of Flanders, Friesland or Brabant claims, I'm sure you can go a lot faster.
After the Kingdom title you are ripe to go after the Emporer of HRE title. But in the meanwhile you can enjoy yourselves with more de jure Kingdom claims. As adviced by other guides, try to keep all the provinces of two full Duke titles, so no vassal count of yours could get ambitious for one of your Duke titles.
@Mods: I'll add a screenshot in the Photo-section soonish, so you can leave it empty for now.