Form a single closed loop out of the grid edges, in such a way that every numbered square has exactly that many of its edges included in the loop.
Click on a grid edge to mark it as part of the loop (black), and again to return to marking it as undecided (yellow). Right-click on a grid edge to mark it as definitely not part of the loop (faint gray), and again to mark it as undecided again.
On a touch screen, tapping an edge cycles it through all three states, so you never need a second button.
You can also play entirely from the keyboard. Everywhere else in this collection the cursor sits on a square; here you are marking the edges between squares, so the cursor sits on a corner instead, and the arrow keys walk it from corner to corner along the grid’s edges. The edge you just walked along is the one your keys act on — think of a pen that inks where it has been:
The cursor is drawn as a green disc on its corner, with a green halo under the edge your keys will act on.
When you have mastered the square grid, look in the Type menu for many other types of tiling!