You would make necklaces and key chains out of plastic craft string by using the same technique would use to make these pieces out of any other type of string. These crafts are fairly simple and easy to learn and would make a great activity for a child. The plastic strings come in many colors and are sturdier than other types of string, so it is more resistant to the elements. These plastic strings are sometimes referred to as "scoobies" or "scoubiedou."
Measure the circumference of your neck with a soft tape measure and mltiply that length by three. Cut two pieces of plastic string at that length. Tie one of the strings at the center onto a small round object like a pen or pencil. Slip the middle of the second piece of string directly under the knot. Pull the two ends of the first string tight.
Start out with the basic straight knot for your necklace. Fold the two ends of one piece of string over the knot and then fold the two ends of the other string over them. Feed the ends of the second string over the sides of the first string and then through the loop of the first string on the other side.
Stop with the pattern after you've reached the length you want for your necklace. Tie a bead onto the end of the plastic strings to act as a toggle clasp so you can easily put the necklace on and remove it later. Make sure the bead is one or two millimeters larger in width than the loop you made around the pencil at the other end. Trim off the excess plastic string.
Cut two pieces of plastic string at the length that you want for your key chain, multiplied by three. Tie one piece at the center on to a pencil and slip the other piece at the middle directly under the knot. Tie the first string tight.
Use the basic round knot to create your key chain or use the basic straight knot. Fold the two ends of one string diagonally over the knot and then fold the two ends of the other string over them and through the loops on either side.
Repeat step two until you have achieved the length you want for your key chain. Tie a key ring tightly to the end of all four ends of the string and trim off the excess. You may need to tie a double knot to make sure the string will not untie itself. You can also use a key chain carabiner or even a large safety pin.