Hi Peter,
A couple ways. I start with the plane on a known flat wood surface. Loosen the knob just enough to allow the blade to make contact with the surface and retighten the knob. If the blade needs correction as to the depth left or right, a gentle tap with a brass hammer on the iron until it is correct.
Sometimes I will loosen the knob enough to wiggle the blade to position while sighting down the bottom of the plane, watching the projection which is evident by the reflection of the lighting along the blade edge.
Take care, Mike