Casting circle is what I would call an essential part of ritual - although others may disagree!
I believe that a circle is needed to help contain your energies until needed. It also creates a psychological barrier from the outside world, which can be very helpful if you're worried about being disturbed.
However, I do not believe that you have to be very formal when casting circle. You can try the chant that I detail below, but don't forget that you can cast circle at any time, whenever you need to do a quick spell.
You can cast a circle just through intent, without any words. Draw the circle with your athame, wand or hand, imagining bright light shooting out and enveloping the area.
If you need a quicker circle, just for a more meditative spell, imagine white light surrounding your body. Glass it over slightly so that it is more like an eggshell or window.
When doing ritual, don't forget to create sacred space first before casting circle. Use a besom, if you have one, or even a vacuum cleaner - it's your intent that's important!
Say something like:
Imagine that the floor is literally sparkling with positive energy.
How I cast circle:
I use my hand to draw the circle, but feel free to use your athame or wand.
Draw the circle, from the direction your altar faces, clockwise. Imagine a sparkling blue light (feel free to change this according to your purpose) streaming from your hand/tool. When you're back at the start, outstretch both arms and draw them downwards. Imagine the blue circle you have drawn flowing down until all 'sides' meet under you feet. Take your arms upwards and imagine the circle drawing up over your head. When doing this, chant the following, if you wish:
'With hand I cast the circle