This is the same game you can find in the games section of StencylForge (search the games area for "Demo-" to find all of my demos). It's designed for you to be able to learn from it, so please feel free to take a look at the coding to understand how I made this game.
To play it is just like any other online word search. You click down on the first letter you want, hold the mouse down until you reach the last letter you want, and then release. If you find all of the words, you can go to the second search (although there's only two in total for the demo).
If the game starts to get laggy, there is a quality trigger button that you can press to toggle on/off the highlighting of found words.