when you capture a fief, you get the choice to take it for yourselvf, or to wait and make a decision later, if you chooce the latter, you can assign the fief by talking to your minister

As posted before on the main board

"@Chuck_Tesla Talk to your companion you have assigned (or your wife etc etc you replaced them with) They are ONLY in your court (your capital town, the one you moved a court to, or your first town) unlike your other advisors who are in any town you own. When you speak to them, you may pick to give a vassal a fief."