Screws. One each on the north & south sides or on the east & west sides. Depending on what you are using them for, they need to be sort of close - more than an inch or two or else seedlings will bend toward a specific bulb. They cover about a 13" square area, 14" at most.
I'm using two of them to supplement sunlight for a couple of banana plants and used them a couple of years ago for tomato/pepper/cucumber seedlings. Work great, though again, very limied coverage area.
YMMV,
Mike