You use it just like any other external meter. You should search "external light meter" on APUG and find plenty of information or go to the "Exposure Discussion" section of the Forum and walk through.
As this is a selenium meter my advice is to pay attention to lateral light. When you meter something in shade in front of you, and you have the sun laterally, you must shade the light meter with your hand as the small lenses in front of the selenium will catch lateral light and you will end up with the typical back light underexposure.
Modern light meters with a silicon cell have the cell "recessed" in some sort of a funnel so that the light can be read from a narrower angle (such as 20° or 30°) without being fooled by a lateral sun.