I am new to the game. The few tutorials I checked didn't help me understand it, or even mentioned more than the fact that I can make them. How can I do that?
You use the Grant Landed Title diplomatic action to make a mayor, bishop, or castle holder (baron). To use this diplomatic action, find the character to whom you want to grant the title. From there you can either right-click the character and select the conduct diplomacy option or navigate to their character page and click the conduct diplomacy (a little circular button with a scroll icon).