For thousands of years occultists have used magick circles for protection and spell casting. They will often have symbols such as pentagrams and spirals in. A magick circle could be used from anything such as a simple curse, to summoning a demon.
Anybody (occultic) can create a circle. Start by drawing a symbol circle on a piece of paper. If you want it to be a good (the good vs. evil good) then draw a pentagram inside it (filling up the whole circle). If you want it to be evil, then you dram an upside down pentagram on it (with the single point facing the bottom). That is called the inverted pentagram. It is often seen with a rams head inside it. Write down what you want to do (maybe make up a spell or something), put your hand over it and say the spell, and if you truly believe that it will work, then it will.
To see some magick circles, click this here.
If you are summoning a demon or something big like that, then it is necessary to cast your circle:
I am not prepared to take the risk of putting instructions of how to cast a circle on my website, in case children then go and do it, and kill themselves in the process.