The surgeon has a few different techniques they might use. 

They might drill tiny holes or otherwise irritate the bone beneath the damaged cartilage, to stimulate healing.

In another technique, the surgeon might remove some healthy cartilage from your bone and grow it in a lab to make more of your cartilage cells. The cells are then implanted into the damaged place. 

A third technique is to transplant cartilage from one place in your joint to another place. Donor cartilage from a cadaver may also be used.