Learn enough historic detail to paint a realistic setting. For example, during King Richard's reign, fierce northern tribes known as the Vandals and Goths sacked major civilized areas. As the countryside settled, every man's goal was to own land. The strongest warrior made himself lord of the conquered land. He retained a large sector for himself, and divided the remaining parcels among loyal followers. In return, these faithful men agreed to fight for and pay taxes to him. Every man living on the land owed the lord some service in return. The men serving the lord were his vassals.
Even though lords were usually noblemen, a lord could also be a vassal of a larger landowner and more powerful lord. The land dividing process continued to the smallest fragment, but provided the means for every man to have something they called their own (even though in reality they didn't "own" the land). The cost for the land was loyalty and service to the lord.
Becoming a Vassal
When a man agreed to serve as vassal, it wasn't by force but a willing agreement. A vassal knelt before the lord, bareheaded and unarmed, during the homage ceremony and agreed to pay homage. The homage ceremony ritual required the vassal to place his hands in the lord's hands and promise to be "his man." This worked like a signed legal document and meant he would fight for him.
The vassal did homage to receive a fief" "a fee or feud held of a feudal lord, a tenure of land subject to feudal obligation." The estate of a feudal lord would be called a fiefdom rather than a kingdom. A fief could be land, a position or the granting of a special permission. As long as the vassal continued his service, the land belonged to him and his heirs. This system of granting fiefs (or feuds) is known as the feudal system.
In this fiefdom, the king was the only true owner of the land. All of the lords under the king were tenants of the land but not owners. Because of this, the lord/vassal relationship was based on loyalty, service and protection. The feudal system bound them all together. If the lord lost his land to another, the vassal ran a high risk of losing his home as well.
To protect his land, a lord would command a small army comprised of vassals who owed him military service. The lord also required his vassals to attend his court and left the enforcement of laws to his vassals. Every aspect of life intertwined the lord with the vassal. One could not exist without the other.
Lords gathered wealth by requiring payment of tribute. Tribute included payments on marriage of the lord's daughter, knighting of his eldest son; and tax upon a vassal who inherited a fief. Lords had the right to collect tolls, and duties on merchants traveling through his land. Unlike taxes today, the lord could not continue to raise new taxes, for no new tax or obligation could be levied unless agreed to by the vassal once the homage ceremony had taken place.
Vassals usually retained the right to tax within their lands. Vassal's rights were made clear at the homage ceremony. It made the agreement clear to both sides. Each understood what they could or could not do. If the lord violated his agreement and tried to exceed his authority, vassals had the right to rebel.
The feudal system included many classes of people. Nobles considered themselves soldiers and above working with their hands. A child born to a noble enjoyed the same freedoms as his parents because a child was born into the same class as his parents. Only nobles could become knights, barons and earls.
The eldest son of the noble inherited the manor (a self-sufficient country whose capital was the lord's castle). If the lord didn't have a son, the manor would be divided among the lord's daughters and the wealth used to pull together a good marriage arrangement. If a son did inherit the manor, the father often wed his daughter to one of his knights and reduced the harshness of his son-in-law's required service.
Applying It to Fantasy Writing
Use history to enrich your fantasy world. The feudal system is ripe with possible scenarios involving rouge vassals, jealous daughters that don't inherit the manor, and on the flip side stories of lords and vassals that become true friends willing to fight to the death. When writing fantasy, include enough reality for readers to latch onto as they enter your new magical reality. In a world where loyalty makes it work, divided loyalties make for an interesting plot. Look for threads in history that interest you, and weave them into something fanciful.