When playing Muslims, depending on the faction:

Almohads must knock out Spain quick as possible and if necessary Aragon as well, securing up to Navarre as soo nas they can. If Spanish get any time they will come at you hard and surely be backed up by Crusades from the like sof the French and HRE. The sooner you knock the Spanish out the better your odds.

Egyptians/Turks must grab Syria, as Katank notes, from each other quick as they can. Egyptians are best suited for this as the Turks will be attacked by Byz 9 time sout of 10, it's a sure thing more or less, so don't be afraid of any retaliation. Turks have it very hard, they are in my opinion one of the most difficult and challenging factions to play. Turks start with Syria so moving in troops ther eis a god idea, possibly best to build them there yourself as weakening borders gives Byz incentive to come at you and they are mighty guys at the start. One point to exploit would be that the Sicilians are likely to attack Naples from them so prehaps use that as a way to launch your own offensive while their distracted. A key to beating Byz is to avoid their high ranking generals, defeating lower ones and taking provinces will lower influence and morale/loyalty then pulling out yourself before a counter attack comes and launching another into the province the good general is coming out of. This will give you your best as it is liekly to eventually trigger a civil war in the Byz empire if you do it enough, and you clean up afterwards, at least this works for me. Also, your main advantage is to hire mercenaries double quick, as it'll give you good strong troops straight away that you can use to launch your assault. The cost is very steep so be careful here, if a unit is beaten down to about 3 men then disband it, it's just taking up space. Smae time manufacture as many of your own home grown guys as you can.

Good luck.

P.S - If any of that is wrong please tell me, it seems to work for me, just.