It works in a silimar fashion to the Ilford EM10 meter. You setup a representative negative in enlarger and find a good print time using test strips. Then without changing aperture and with safelight OFF place the sensor under a light part of the image (ideally you are selecting a part that you want to be shadow with a hint of texture) press button on side to turn on (it needs a battery) - turn dial - there are two red lights and one green - note reading when green light shows - this is your paper speed - write it on the paper box along with exposure time you found. To print another neg perhaps at a different enlargement, with safelight off, set the dial to the right speed for paper, open up enlarger lens to widest aperture, place sensor under a light part (same idea looking for a shadow with slight detail) press button on side and adjust aperture until green light appears - your exposure time will be about the same as you wrote down. I use my EM10 to get me in the ballpark for exposure time and then use small test strips to fine tune.
I think as no green light on this it would be when both red lights on !