Step 3: Poke Holes in the Lid
It might help to have more than one nail or thumbtack, as they get rather dull after clashing with so much metal.
I used a thumbtack for my first stove, but grew weary of pressing my sore thumb pad against a tack. So for the next two I made, I used a hammer, a finishing nail, and a block of scrap wood. This allowed me to punch the nail through the lid, into something that I was willing to tear up, and then pull the nail out and make another hole.
As far as hole patterns go, make sure that the holes are not too spread out, and that there are enough of them for good flow. If you have few holes spread sparsely across the lid, it will not heat the tin up enough to boil the alcohol in it and keep the stove going.
Make sure the holes are dense in the center. Do not put too many holes either, or it will turn into a raging inferno capable of burning rapidly through your fuel.