It was a few years ago I made this for a fundraiser auction for preschool. I used a 1×6 pine board and one inch nails. After staining the wood and letting it completely dry, I traced a printout of the jar design onto the wood with chalk. The hardest part is definitely getting the nails even and straight. I used a smaller than usual hammer which made things a little easier too. Here is a great blog with step by step directions for beginner String Art.
A Patch Of Inspiration 