Visualization of Circles
A mark of true knowledge and power is the rejection of all that is not necessary (the principle of Occam's Razor). In most circumstances a physical Circle is not necessary and, in fact, reliance upon a physical medium for expression of an arcane principle will have the effect of reducing its effectiveness.
It is best, except when dealing with the most serious or dangerous of conditions, to visualize a Circle as a closed sphere of the desired size. It is very helpful to include with such a visualization an appropriate geometrical figure (such as a pentagram, triangle, square, etc. ) with its vertices congruent with the Circle. The figure used will depend upon what your purpose is, and there is no reason not to use more than one. As an example, if a ritual requires that the four elements be included, you will want to visualize a square.
Color symbology is very important when using such visualization techniques, and, as is true of nearly all symbols, the colors you use may be very personal in nature. They should, however, be consistant with your purpose.
Admittedly, the perfection of any kind of visualization technique requires a certain amount of practice, but anyone can learn it. It is best to begin with a constructed Circle, superimposing your visualization upon it, until you are satisfied that you can hold the visualization in spite of the distractions which will accompany the Circle's use.