How to Draw a Perfect Circle Using a Pin
To draw a perfect circle using a pin, start by cutting a piece of string that's 1 to 2 inches longer than your desired radius and tying a small loop at each end. Then, place a piece of paper over some cardboard or styrofoam, push a pin through the center of the paper, and loop 1 end of the string over the pin. Next, loop the other end of the string around the end of a pencil, and pull the pencil away from the pin so the string is completely taut. Finally, place the tip of the pencil on the paper and draw a circle, keeping the string taut as you go.
Cut string that’s 1–2 inches (2.5–5.1 cm) longer than your desired radius. The circle you draw will have a diameter that's twice as long as the string you use, but you should cut it 1–2 inches (2.5–5.1 cm) longer than you want at first since you'll lose some of the length when you tie a loop at each end. For example, if you want your circle to be 4 inches (10 cm) across, then your desired radius would be 2 inches (5.1 cm) since the radius is equal to half of the diameter. Therefore, you'd cut the piece of string to be about 3–4 inches (7.6–10.2 cm) long so you have extra string on each end to tie the loops with. Tie a small loop at each end of the string. You'll use one of the loops to attach the string to a pin and the other loop to attach it to a pencil. The size of the loops doesn't need to be precise, but make sure they're big enough to fit around a pencil and pin but not so big that they use up a lot of the string that you cut. When you're finished tying the loops, you can measure the length of the string to determine what the exact radius of your circle will be. If the radius is too short, you can always cut a longer piece of string and try again. Place a piece of paper over some cardboard, styrofoam, or cork board. Any kind of material will work as long as you're able to press a pin into it. This base under the paper will give the pin something to stick into and help hold it in place as you use it to draw your circle. If you're drawing your circle onto a piece of thick material, like foam poster board or canvas, you can skip this step. If you don't have anything to use as a base under your sheet of paper, you can try holding the pin in place on the paper with your fingers instead. Push a pin through the paper where you want the center of your circle to be. Any kind of push pin will work, although the shorter the pin, the easier it will be to work with (since a tall pin might stick up more and be less stable). Make sure it's not too close to the edge of the paper or else your circle might go off the page when you draw it. Loop one end of the string over the pin. Don't worry about tightening the loop. As long as it's wrapped around the pin, you're all set. It doesn't matter what end of the string you wrap around the pin. Just make sure the loop on the other end is big enough to wrap around a pencil. Loop the other end of the string around the end of a pencil. Simply slide the tip of the pencil into the loop so the string is wrapped around the pencil. It's OK if the loop slides down the pencil. You'll be pulling it taut later on so it stays in place. You don't have to use a pencil to draw your circle—you can use a pen, marker, colored pencil, or another drawing utensil as long as the loop fits around it. Place the tip of the pencil on the paper so the string is pulled taut. Pull the pencil away from the pin until the string is completely taut. Then, place the tip of the pencil down. This is where the edge of your circle will be. Don't pull on the string too hard or you could pull the pin out. Draw a circle with the pencil, keeping the string taut as you go. To draw your circle, slowly move the pencil in a circular motion around the pin in the center, holding the pencil up straight and not at an angle. Keep gently pulling the pencil away from the pin in the center so that the string stays taut and your circle is even all the way around. If you don't keep the string taut, you won't end up with a perfect circle. If your hand slips and you make a mistake, just erase what you've drawn and start over. Remove the pin when you’re finished drawing your circle. If you want to draw another circle, you can push the pin into a different spot on the paper and repeat the process. Otherwise, set the pin and string aside for the next time you need to draw a perfect circle! 
Cut string that’s 1–2 inches (2.5–5.1 cm) longer than your desired radius. Tie a small loop at each end of the string. Place a piece of paper over some cardboard, styrofoam, or cork board. Push a pin through the paper where you want the center of your circle to be. Loop one end of the string over the pin. Loop the other end of the string around the end of a pencil. Place the tip of the pencil on the paper so the string is pulled taut. Draw a circle with the pencil, keeping the string taut as you go. Remove the pin when you’re finished drawing your circle. 