Easy but messy...............
Put some grease in the palm of your left hand if you are right handed.
Now with your right hand holding the bearing on one of its edges squeeze the grease into the big end of the taper bearing as if you were Knocking on a door using pressure to squeeze the grease in.
Slowly rotate the bearing as you go, their will be grease oozing out from between the taper roller bearings as you go.
And have a clean rag handy.