To cast a circle is to place a protective energy barrier around you and anything else you need to have in the circle with you.
To start your cast you should ground and center then gather some energies to use for the casting. Pick up the tool in which you want to use for the casting. This can a wand, knife, finger, etc. You may start in the north or east depending on your liking. There you will cast the energies you gathered into a ring around you. Point your tool outward and walk clockwise around your circle moving through each direction. You may need more than one pass depending on what you feel. You can also do this in your mind when there is no room to walk the circle.
To open the circle back up take a broom and sweep the circle open moving in a clockwise manor.