If you already have 8ft fluorescent bulbs, the Retro-fit LED's are a killer upgrade. They bypass the ballast, but it's EASY to wire!
There is a white wire and a black wire coming in from household current. These are your hot wires. May also be a ground. The black wire goes to one end of the bulbs, and the white wire goes to the other side. It's really that simple.
You can use the existing wires in the fixture. Cut all the wires going in and out of the ballast. Then use a wire nut to connect the white wire to the ones running to one end... and a another wire nut to connect the black to the ones going to the other end.
It does not matter which end the white or black wires go to... as long as one goes to one end, and the other to the other end.
Install the bulbs, and enjoy twice the light for half the electricity. As a bonus, they come on with instant full brightness, regardless of the weather. I've wired 40+ fixtures. Each one only takes a few minutes. Each bulb puts out more than TWICE the light as a normal fluorescent... so if you want to be super cheap, you can only install one bulb in each fixture, and still have more light than you did before. But as we get older, the more light the better! lol
The only other decision is clear or frosted covers/lenses. Clear is fine if they are way up high, but if they are within your field of vision, they produce an intense glare. Frosted covers are far easier on the eyes... and still work great when they are high up.
You can also get clips that install with a self tapping screw that keep the bulbs from sagging in the middle. Recommended for 8ft bulbs.
For 4ft fixtures, it's sometimes cost effective to buy the self contained units. But, if you have 8ft fixtures, these Retro-fit bulbs are amazing. FAR better light, easy to install, and you don't have to go tearing down and re-installing fixtures.