Turn off the light switch
Turn off power at the breaker if possible
Let the bulb cool completely
Place a stable ladder or step stool under the fixture
Remove the light cover if there is one
Hold the bulb firmly with both hands
Rotate the bulb a quarter turn until the pins align with the slots
Lower the bulb straight down
Insert the new fluorescent bulb into the sockets
Rotate the bulb a quarter turn until it locks in place
Reinstall the light cover
Turn the power back on
Test the light
Dispose of the old bulb properly according to local rules