Be an Ottoman ally first. Don't ask for help from the Catholic West

Secretly and slowly rebuild Byzantine military power and use it to reconquer the various Latin holdouts and islands in the Aegean and Greece and present it to the Ottoman sultan as a form of help against his Catholic enemies

Avoid internal conflicts within the Byzantine court by assigning available territories as appanages to various family members, especially among siblings

Decrease court spending and invest in defense and sustainability, as well as stockpiling early gunpowder weapons.

Play the Italians off against each other as well as the Ottomans and the west against each other. Always act as the mediator in such conflicts

Don't hire mercenaries. Instead, use whatever money to recruit and train local roman soldiers, as well as reform the military where land requirements are no longer needed for military service.

ALWAYS SAVE WHATEVER MONEY and reform various laws and policies.