Locate and mark the ceiling joists with a stud finder.
Shut off the power to the existing light fixture. Unscrew the base with a screwdriver to remove the fixture. Remove the wire nuts from the top of the wires to disconnect the wiring. Set the wire nuts aside for later use.
Lift the square canopy for the fluorescent light to the ceiling. Screw the canopy to the ceiling joists. Secure light fixtures that don't match up with the ceiling joists with toggle bolts.
Find the metal knockout in the top of the fluorescent light canopy. Push the metal piece from the hole with a screwdriver. Feed the wiring from the electrical box through the hole.
Connect the wiring for the light fixture with the wiring in the ceiling electrical box. Twist together the bare metal ends of the two white wires. Twist a wire nut over the wire ends. Repeat with the black wires. Twist the green grounding wire around the grounding screw in the fixture.
Slide the 4 fluorescent lighting tubes into the light fixture so the ends of the tubes touch the sides of the fixture.
Snap the light diffuser panel into place per manufacturer directions. Turn on the power to the light fixture.