As it was said before, any knight can make another knight, and only the king can make a lord. Enfeoffing land should go along with this guideline. If a great lord has some extinct vassals, he can grant their lands to a knight (or several knights, if it's too much for one knight), but if he wants another lordly house, he has to petition the king for that.
Knights can be made and enfeoffed with land any time. Even a landed knight can, in turn, enfeoff some land to a sworn landless knight in his service. Ser Eustace Osgrey could, if he wanted, give one of three his villages to Dunk, creating a very tiny House Tallman of the Reach.