One important thing i keep in mind EVERYTIME i play Byzantium is if Hungary and Poland doesn't rival eachother and instead rival Ottomans. If they rival eachother i restart since Ottomans can pretty much rekt you with only 2 armies. Another thing that i think is worth a restart is if the stupid poles does not take the Personal Union with Lithuania.

So Poland-Lithuania and Hungary is needed pretty much to win the war against Ottomans. Now keep in mind you can still keep restarting 20k times because the Ottomans attacked you before you could get alliances. If the Ottomans attack everyone else (Albania, Candar, etc) then you have time for the making of alliances. One thing that can screw up everything is if Hungary rivals Lithuania, Aragon and Ottomans. If poland take the PU he will rival Poland, So try to get a Hungary that doesn't rival any of the SLAV SQUAT BOYS ie Lithuania/Poland. The reason you need your future allies to rival Ottomans is because you get a pretty big modifier towards alliance "+20 threatend by Ottomans" but only towards those who rival Ottomans. So just improve relations with Hungary and Poland until you can ally them. After Poland gets their PU they usually attack the Teutons, so it's worth waiting it even if their truce expires with Ottomans (1450, January). You might need to send a gift but i find that not needed. TIP: If Ottomans take some time before they declare war (more than half a year) on Albania, you will have enough time to get alliance with both Poland and Hungary.

So in 1450 January the truce with Ottomans and their enemies (Poland, Hungary etc) ends so you can declare war. Keep in mind this before declaring war though, that you want the Ottomans to fight in the Anatolia/Asian part of his nation. It's alot easier to siege everything when their troops are on the other side of the bosphorus.
If you want it easier, do not let the Ottomans take serbia like they did there. So just siege the forts and everything and make the peace deal you want.
I went for something like this.

The Anatolia part i got in my second war with the Ottomans but look at the balkans to see what i captured. I vassalised Wallachia and made them a march in my game and i kept them through the whole game, they gave me like 22k troops in the end. When you have taken your stuff you want in the balkans, remember this: Your allies won't help you if Ottomans don't have stuff nearby so leave their stuff there in the balkans. In my pic above i left those ugly green ♥♥♥♥♥ on the balkans for almost 150 years just so all my allies would help. I got a coalition against me after my Anatolian conquest and after my truce with Ottomans ended he went at me too with all of his muslim friends. The coalition strikes, we win, Ottomans are dead and i wrap up them in a chapter they should have been in real life.

So there are pesky Italians on your doorstep, just like 300BC, So called Genoa and Venice. Now they own quite a bit of land that is GREEK culture, Negroponte, Naxos, Corfu and Scio/Chios/Lesbos and Caffa but it's Pontic (but you need this province for the Basileus Achievement) so to attack them can be tricky since they are strong nations with usually strong allies
(Genoa usually allies with France and Venice sometimes other strong nations.) To attack Venice you can usually attack The Knights' and NOT COBELLIGERENT Venice since he will call his buds in. Im only going to take stuff from Venice in this war and leave The Knights' open so i can grab him later after im done with the Venetian ♥♥♥♥. To attack Genoa can be tricky, i had to wait until his ally France couldn't help them in the war and then i attacked. France couldn't help since he was getting pesterd by Austria and England. I attack Genoa and take Scio and Caffa quite easily.

By this point i owned half of Anatolia, but that isn't enough for the glorious Basileus (achievement).
Because you killed the Ottomans, the people left on anatolia is most likely Candar, Karaman, Mamluks and probably some other nation (in my case a Shia Trebizond). Mamluks are most likely going to ally Crimea or someone else up north and there pop goes the weasel you are going to attack Crimea and Poland WILL help unless he is ♥♥♥♥♥♥ by 100020 thousand nations at the same time. Poland will help you crush Mamluks and their friends in the Crimean war (remember to co-belligerent Mamluks) and take as much as possible. Now your allies are pretty much only for the defensive part and not offensive but now you should be powerful and able to fight wars by your own.
this is how my nation looked like when i got Basileus achievement.

So to leave it like the last picture is quite boring, i think we need to make Jerusalem christian or precisely Orthodox. If you own everything on the last picture, you should be able to win against the mamluks by yourself. Check the ledger and if you have more troops, ATTACK! Hopefully you win the war easily.
I took Antioch and that coastline all the way down to Jerusalem

I also got 2 events for taking Antioch and Jerusalem
Now i only took the coast and i will take more inland but i focused on the mammyluk coast first.

i took this amount of coast in my wars (2 so far) and then i took Egypt in my third war. I got forced into war with the arabian nations because the took the rest of the mamluk levant.

Which gave me mecca and other parts too.

Now to kill the arabians is quite an easy task since their troops suck compared to yours late game. and this is quite funny to bring christianity to arabia i dont know what to say anymore hahhahahhaha no but really do what you want after this, i chose to invade italy but the stupid french put me out but seriously by this point you should do whatever your heart wishes because you are probably going to be powerful as ♥♥♥♥

this is my nation in the end of the game
Hope you enjoyed the guide, have a good day and good luck.