The Kingdom of Serbia is small kingdom among great enemies. In the north and west there is the Kingdom of Hungary, and in the east and south the Byzantine Empire. No matter how you look at it, at some point you will be at war with at least one of these countries.
Everything started in 1112 when Uros came in power in Rashka. With him new dynasty rose up in power Nemanjici. Beside Rashka he is also an owner of county of Hum and he is not a king still only a duke. To create kingdom of Serbia he needed in his power county of Belgrade which is in Byzantine power and duchy of Duklja which consists of two counties Zeta and Ragusa.
To create this kingdom you must start from the weakest enemies and that is Duklja. Claim Ragusa as soon as you can, and declare war when you are ready. After taking Ragusa there will be some more tough choices to make. Once the claim on Ragusa war is over, you will be at peace with Zeta so you can concentrate your attention on Belgrade now this all depend on situation in the Byzantine Empire. If the Byzantine lords are fighting some inner wars and there are many new states created there is a good chance since you start with de jure claim on Belgrade.If Belgrade is independent that is big plus and if they emptied their fort levy that is even bigger. Of course this is two edged sword since everyone in the empire is now aggressive and there is also a chance that they will sign peace and gang up on you and there is no chance to fight an open war with the empire. You have to besiege holdings and take them. Unfortunately you will have to trust in luck. Sign peace as soon as you can and claim those holdings. It is better if they are independent because than you gain it for you but if they are the part of B. Empire their lord will become your vassal, which can lead to future trouble. Now all you have to do is take Zeta. As a reason for war I used de jure claim to create kingdom. That war was quick and easy. When I had money needed to create title I did it, but first you must create duchy of Duklja and than Serbia. That’s how I become king of Serbia quickly during 40 years.
This was a big talk about creation of kingdom of Serbia in game so here is some history stuff. Stefan Nemanjic I son of Stefan Nemanja became king of Serbia in 1217 the pope Honorius III gave him the crown. With it Serbia finally became fully independent state.