Cut off the power to the lights you will be working on at the circuit board. This is essential to avoid electrocution.
Remove the plastic lens cover that covers the lights. Snap off the plastic cover carefully to avoid cracking it.
Remove the existing lights by twisting them a quarter turn and then pulling down. These lights contain toxic gases, so dispose of them according to local laws.
Locate the existing ballast. It is the large black box with wires running to it.
Cut the wires running to the ballast with wire cutters. Strip off about one inch of insulation to expose the copper beneath.
Unscrew the T12 ballast while holding on to it to keep it from falling when it's unscrewed. Set it aside and dispose of it properly. In most municipalities, these ballasts can be recycled at the local recycling collection center.
Install the new T8 ballast by screwing it into the same place as the old T12 ballast. It will be roughly the same size, but you may need to drill new holes to accommodate the different screw positions.
Wire the new ballast with the existing wires according to the instructions included with the ballast. Tape off the connections with electrical tape to keep the connections from touching each other and shorting out the light.
Install the new T8 bulbs. Though these bulbs are thinner, they will fit in the existing fixtures, as they use the same pin configuration as the old T12 bulbs and install the same way.
Snap the plastic lens cover back into place carefully.
Tun the power running to the light back on.