This one's not too difficult but relies a lot on getting a good starting position.
Setup:
1. Set max possible counties to highest and number of duchies, kingdoms, and empires to None.
2. Use historical de jure kingdoms in order to find good starting positions easily (a 3-province count in Wales is a much better start than a 5-province count in France)
3. Other stuff can be as random as you like depending on your playstyle
4. Your choice of starting count will depend on your play style but things to look for are:
4.1. Large number of starting counties (3-5 is good)
4.2. All counties in same de jure kingdom
4.3. Small de jure kingdom (Ireland, Scotland, Wales, Flanders, and Denmark are all good candidates)
4.4. Multiple religions in de jure kingdom and/or tribal government a plus for easy expansion
4.5. Religion/government that allows raiding a plus (for gold)
Keep generating until you get a good start. With a good setup like the above everything should be pretty straightforward. Declare war as early and often as you can; religious wars are great if there isn't a strong follower of that religion near you since county conquests cost piety. Borrow from merchants and raid neighbors to get the extra gold you need and have your Court Chaplain build zeal for piety (if tribal).
Starting as a four-province count in Ireland I was able to found the Kingdom of Ireland in 13 years without any real difficulty. The only impediments I ever had were waiting for the 50 piety between county conquests and amassing the ~350 gold to found the kingdom at the end.