There is an easy way to create a perfect circle on your wall. Use a circle as an art piece on its own, or use it as part of a larger piece.
Tie one end of a string to a thumbtack.
Tie the other end to the tip of a pen or pencil. The length of the string will be the diameter of your circle so make sure this is the length you want. If you want the circle larger, make the string longer; make it shorter for a smaller circle.
Press your thumbtack into the wall where you want the center of the circle to be. Don't press it tight against the wall, but rather leave a small space so the string can move around under the thumbtack. Pull the string tightly in one direction.
Keeping the string pulled tight, begin drawing your circle by using the thumbtack as an anchor. Make the line dark enough to see clearly and try not to lift your pen or pencil or your circle will not be accurate.
Begin painting inside the circle using the small paintbrush. Get as close to the line as you can and cover up any dark pen marks with your paint.
Use the larger brush to fill in the rest of the circle.