At first, vassalize Serbia, then Bosnia, put your army over maximum and get good alies.
One thing though - AI often warns you not go to war, but this is very inconsistent. When it warns you at start (while he is at war with Albania), it has never any effect - you just go and conquer Serbia and Bosnia. But I lost 2 times just because I tried to conquer Wallachia lately - both times I thought that Ottomans would be too busy with their wars, but, surprisingly, both times they responded and attacked me. I don't understand this behavior - AI does not attack you despite of having cores, but when you attack someone under warning, AI joins, even though it has its war going.
Overall, it is real, but I always have feeling that my troops are illusionary. Ottomans just wipe them out.