Open or remove the fluorescent fixture's light diffuser cover, if applicable. For hinged diffuser frames, rotate the thumb tabs located near the ends on the opening side of the frame horizontally outward to open the frame. Pliers can be used if the tabs are difficult to turn. For compression-type diffuser covers, grip the side of the cover and spread it outward slightly to release it from the fixture on either side.
Remove the old fluorescent bulbs, if applicable, by gripping the glass tube of the bulb near each end and rotating it 1/4 turn to free the end pins from the porcelain sockets at each end.
Hold the T-12 fluorescent bulb horizontally beneath the light fixture and align the ends of the bulb with the fixture's porcelain sockets, making certain the end pins on the bulb caps are positioned vertically.
Raise the T-12 fluorescent bulb vertically to insert both end caps of the bulb into the porcelain end sockets, then when fully inserted, rotate the bulb clockwise 1/4 turn to lock it into the sockets.
Replace or close the fixture's light diffuser cover, if removed or opened in Step 1.