Retrofitting a light fixture to an LED light engine is a relatively straightforward process that can help to save energy and money in the long run. Here are the general steps to follow:

1. Determine the compatibility: First, check if your current light fixture is compatible with LED light engines. Look for information on the fixture's packaging, or check with the manufacturer to determine whether it can handle the electrical requirements of LED lighting.
2. Choose an LED light engine: Once you know your fixture is compatible, choose an LED light engine that matches the specifications of your existing fixture. LED light engines come in a variety of sizes, shapes, and color temperatures, so select the one that best suits your needs.
3. Turn off the power: Turn off the power to the fixture at the circuit breaker to avoid any electrical hazards.
4. Remove the old fixture: Remove the old light bulb, ballast, and any other components that are no longer needed. Depending on the type of fixture, you may need to unscrew or unclip the housing, or detach the light socket from the fixture.
5. Install the LED light engine: Install the LED light engine into the fixture, following the manufacturer's instructions. In most cases, this involves attaching the LED engine to the existing socket or wiring, and securing it in place with screws or clips.
6. Test the new fixture: Turn the power back on and test the new fixture to ensure that it is working properly. If there are any issues, double-check the wiring and connections to make sure everything is correctly installed.

That's it! With these steps, you can easily retrofit your existing light fixture to an energy-efficient LED light engine, and enjoy the benefits of long-lasting, high-quality lighting.  Always call a qualified electrician if you do not feel you can safely install the lights.