Put them in the pot and boil till soft, add enough water so as the water in the yam won’t dry up because you will need it while pounding.
Scoop the hot yams into a mortar and pound with the pestle, if the yam that you are pounding gets stiff add some of the water used in cooking the yam into the mortar and keep pounding.
Pound the yam until it has a stretchy consistency and doesn’t stick when touched.