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.

hello there! Can you explain to me how did you come out with that pattern? How did you figured out which light to turn off and in what order ?