Cork trivets (These came in inexpensive three-packs from Ikea)
Tack hammer (optional)
This project can also be done using a wooden board in place of the cork. With the cork, we found the kids (ages 9 & 10) could push the nails in with their fingers instead of using the hammer (though they seemed to like tapping with the hammer anyway) but then there is an increased chance of little smashed fingers. This was as an activity during my daughter's birthday party so we opted for the safer cork.
In order to see the string design better we chose to use a black background. Since we already had a big bottle of black poster paint we used that. We used water-soluable craft varnish to finish it off and give it a slight shine. The poster paint alone looked very dull.
After the paint was dry, my daughter picked out a design (in this case the letter "h" from her name). She copied the design with nails. Once she was happy with the placement of the nails she began winding different colored embroidery floss around the nails. There really is no right or wrong way to do this and it is great for experimenting with different patterns.
My 10 year old was able to do this entire project by herself. The most challenging part was tying the knots of string around the nails. A younger child may need help with tying off of string.