Physically determine the bounds of your circle, if you want. Do this by drawing a circle on the floor, sprinkling salt water in a circle, or by laying a cord in a circle (make sure you tie it). Anyhow, the circle is usually as wide as you are tall.
Place all the objects you will be using in your ritual inside the circle.
Represent the elements in the four quarters of your circle (called corners by some, but a circle doesn't have any corners!). Place something that represent Earth in the north, Air in the east, Fire south and Water west. Bless (or ask for consecration of) these things. (See tips for things that represent the elements.) Be sure to have some food and drink to share with the spirits, to thank them and to replenish your energy.
Take your magick weapon, whether it be hands, a broom, athame or wand, and walk round the spot where your circle will be thrice, counterclockwise: "Cast the circle thrice about, to keep the evil spirits out." State the purpose of the circle.
Invoke the elements and spirits and deities you wish to work with. Call them to the objects that represent them. Take each of the objects that represent the elements around your circle, filling it with the power of each of them.
Meditate for a while now. Meditation or astral projection might be the main act, or it serves just to shift your consciousness.
Do whatever it is you want to do in your circle. Remember: "An it harm none, do what ye will." If you must leave the circle before you are done, cut a doorway (quite literally, envision a doorway being cut from the edge of the circle, up, to the side, then down again. Reclose the circle once you're back in.
Close the circle when you're done: pay respect to whatever deities you invited, thank the elements before you remove their representative objects, and finally undo the casting by casting in reverse.
Experiment with different ways of casting. This is only one way. It should be comfortable for you: change whatever you want.
Salt, a stone, or a green candle could represent Earth, incense, broken glass, or a yellow candle for Air, water (in any receptacle will do, you could use a silver chalice or an empty cat food can) or a blue candle for Water, and a red candle or cigarette for Fire. If you have one, you could also use the Aces in your Tarot deck.
things you will need
Water, Salt, Candles (Red, Blue, Yellow and Green)
Book of Shadows