Visualize a line of flickering blue flame, like a gas flame.
Now mentally draw a pentacle in the invoking direction, starting at the top and moving down to the left. Watch it form out of the blue flame. Hold the image in your mind for a few moments.
Now retrace it is the banishing direction, starting at the lower left-hand corner and moving up. As you do so, watch it disappear.
Practice until it comes easily to you. This is part of casting a circle.