Reviewed - Chainring Power Meters
Any reasonably competent self taught bike mechanic should be able to fit these. If you've every changed your chainring before, you should be fine. If that's not you, I'd suggest paying your LBS to fit them. These units measure the power from the right leg (drive-train side). Like many power meters they provide average total power by doubling the power measured in this one leg.
Some units, like the Powertap C1, provide estimated left & right power balance. They do this by attributing the power on the down stroke to the right leg and the upstroke to the left leg. It's not absolute but it's consistent.