Depending on your canvas size (I used 8x10), print out an outline of your desired state. I love my Mitten and placed the heart over Holland... were I currently live. Sorry upper peninsula... you didn't make the cut. Tape the printout securely over the canvas.
God bless West Michigan.
Start pushing/hammering your nails around the outline in consistent intervals. Until you hit the wooden frame around the edge of the canvas, the nails push in very easily by hand. You'll have to hammer in the rest.
Carefully remove the printout without pulling out the nails.
I was originally planning on just wrapping the string around the nails. However, I didn't think things through and used nails with tiny to non-existent heads on them. Awesome. This was how I remedied things. I removed the nails that weren't nailed into the wooden frame. Using a needle and thread, I "sewed" the string in a random order through the existing holes. If you go through a hole, come up through the one right next to it to fully utilize your string and not have a mess on the back.
For the nails that were already permanently in there, I carefully wrapped the string around it tightly several times. Regardless if you thread the string through the hole or around the nail, keep the string taut.... or you will be sorry.
So you don't lose your mind, do not anticipate using one long piece of string. You will die. Your mind will turn to mush and I guarantee you'll throw the canvas against the wall. I used many many pieces and just taped the ends on the back.