It's important to have a good relation with a lot of lords in calradia, across the 6 empires, talk to them (if you have a good relation with the lord) and ask him how he feels about his current liege, if he's pissed he will tell you and then you have the oportunity to ask the lord if he's willing to become your vassal. There automatic changes too, which can result in a lord defecting and turn to your empire or they may be kicked out for treason and join your empire too.
Tip: Don't give away more than 1 fief to a single lord, assuming you're at war, you're gonna need more lords and the fiefs you own won't be enough to all of them if you start recruiting them like crazy.