Mostly trial and error using the web-gui.
One way to make make the thinking slightly easier is a few properties of the problem.

1. You only need to find how to turn on the middle light, a diagonal corner light and a middle corner light. Then you can rotate the solutions to get all of the combinations.
2. If you keep track of button presses to get to a specific light combination, you can add two such combinations and do a modulus(2), on all numbers to get a new minimum number of presses to get to the combinations of light switches.

It is a little hard to describe in text though. I recomend pen and paper.