When drawing a magic circle (such as a pentagram), it is important to get the figure right. Drawing a pentagram freehand makes the circle sloppy and inaccurate (so don't do it like in this Youtube video!).
Traditionally circles were drawn using two leather straps or cords, using a method I'll describe below. The method is really simple, but the trick is getting the straps the correct length.
The first strap is called the Radius, and it is very simply the radius of the circle you wish to draw. Traditionally the circle should be as wide as your height. Which means the Radius needs to be half as long as you are tall. The Radius is therefore fairly easy to create: you take a piece of string and measure it out to your full height (lie down and get a friend to measure this out). Then fold the string in half and use this length to cut your strap to the correct length.
The second strap is called the Apothegm (a-PO-thuh-gum: with the PO as in pot, not poke). It is much more difficult to get the right length. Before the availability of tables of trigonometry, a practitioner needed to perform a complex geometrical construction using many straight lines and straps of different lengths*. When the construction was complete, the practitioner could directly measure the length needed for their Apothegm. Contrary to popular belief, this complex construction was not done for each circle the practitioner required (it takes far too long, is too prone to error, and leaves many construction lines that pollute the desired shape), but was carried out beforehand to establish the correct length of the Apothegm.
Fortunately we now can calculate the length of the Apothegm quickly and easily, removing the need for the complicated preparation.
* I won't go into the methods of constructing these shapes here, although I have to confess they can be quite fun to try. This Youtube video shows one occult order constructing a pentagram. I include this for illustration only: theirs isn't a great way to do it, as they don't base the size on the Radius. Wikipedia has an animation of the right way, again for a pentagram. These approaches get far more difficult for shapes such as the 7-pointed star. Often the knowledge of how to construct the Apothegm was passed on as mystical knowledge, along with the rituals that used the final circle. Fortunately we don't have to go through these steps now.