As Usonian says, it is mostly agitation and hot water. You can speed the process by "shocking" the wool, going quickly from hot soapy water to cold clear water.

Most of the fulling on mine, I did by rubbing the wet, soapy bonnet with my fingertips against the bottom of the laundry room sink.

"Walker" a Scottish family name, was actually an occupational name for a person who made felted or fulled fabrics by walking on them while they were wet. "Felting" regards loose fiber, "fulling" is the same process using woven or knitted fabric.

Naturally, fulling works best when it is unintentional, i.e. someone being "helpful" by tossing your hand-knitted sweater into the machine with heavy jeans.