It all depends on what the circumstances of the ritual are. When I am Solitary, I usually setup an altar with all the elements represented and do a visualization of what they mean to me when calling them. After the Quarters are called, I visualize the Circle as a white bubble encompassing the area I am working in.
When working in a group, we usually use formal Quarter calls and envision the respective pentagrams acting as guardians and beacons and the HPS draws the circle symbolically in a deosil direction. Sometimes in a small grouip, we all reach down and pull the circle up.
Then there are times when I need a sacred place in the Astral thatand my circle is a nice cave with a clear water pool in the womb of our Mother.