Take the mission for Albania, kill them quickly. Make sure you bait them into highlands so its not mountains. Kill Bosnia. Ally Ottos rivals, preferably Austria and Hungary. From there you have to wait for either favors or ottos attacking you. If Austria rivals Venice, declare war on Venice promising them Land and just give them enough to gain 10 favors. Its probably not worth taking anything yourself because of revolt risk, but they will prob end up going into Austria so w/e. Once you have 10 favors with Austria you should be able to call in Hungary (based off land, most likely Vidin) and then Austria off favors. Make sure Mil tech isnt a big factor. This generally only happens with Austria because of the 1 mil leader, Hungary gets a really good regency and then a really good leader from an event. This was should be easily won if mil techs are fine Earlier the better because Ottos tend to snowball super hard early.

After first war is one just take as much land as possibly, dont worry about dip points or whatever. Also rebels dont matter because they will just go into Otto land and ottos will kill them. After that its just waiting for moar favors to call in Austria and Hungary. Also Mamlukes turned friendly to me after I rivaled Ottos, so be on the lookout for that. They are pretty trash as an ally but will at worst take up otto manpower and army time, at best win some battles and siege the eastern forts.

Oh i forgot to mention, get a border with Wallachia in first Otto war so you have another easy target. I was in super debt for the first 50 years basically (also got shit rulers), but as with most countries after the first war its easy from there.