Number the tiles like this:

4 5 6 7 8 9

2 - 5 - 8 - 7 - 4 5 - 6 - 9 - 8 - 5 6 - 3 - 2 - 1 - 4 - 5

When you get done, check to make sure all the switches along the edges line up with slots. If not, you made a mistake somewhere along the way. You can either try to correct it yourself -- or click on "Reset All" and start again.