You mean the Ilford EM-10 enlarging meter? I have one, found it a bit of a pain to use. I never use it anymore since I don't do darkroom work anymore.
It works by you making a print that is good, then writing down the exposure time used. Then you place the EM-10 under the enlarger under a a standard tone area (you can choose anything..skin, white, middle grey, etc) then turn the EM-10 dial until the light on the meter indicates correct exposure. Write the number the dial indicated.
In the future, you use the same exposure time and dial setting used to set the EM-10 up, and place in an area of similar tone when making a print from another neg (or the same neg at a different print size). Open and close the lens aperture till the EM-10 shows correct exposure. I found it useful for making different size prints of the same neg, but a pain to do different negs since I never got it under the exact same tone in the new neg as was used to calibrate it.