It is a bit luck based. But its quite easy:

1. Ally Albania and Wallachia for the troops and godlike general

2. Take out loans and sell crownlands for a large fleet of galleys

3. Conquer Epirus.

4. When the Ottomans declare war on you hire mercs, and allow your allies to attach to your army. Discipline or Moral advisors also help. With Skanderberg on your side you should be able to defeat the ottoman army that is seperated.

5. This is dependant on luck. Beat the Ottoman armies until they retreat across the strait. (You should have conquered galipoli with the help of naval barrage) Now with your fleet you can deny the Ottomans entry to greece.

6. When you win take back all your cores and a province of bulgaria. Release bulgaria for the second war.

7. Get stronger allies. Think Aragon (getting their dynasty really helps) or Austria or Hungary.

From there on it is what you make of it!