The altar should be set up in front of you. It doesn't really matter which direction the altar faces, but most prefer that you look east for general purposes. For other purposes you may choose to face other ways.
Light a white candle (this is you altar candle and its best that you always use the same one...of course until it has burnt down completely and you need to buy a new one. This is the only use this candle should have) and meditate or concentrate on what you want to accomplish with you workings.
When you are in the right frame of mind, picture you circle forming from the point directly in front of you. This can be on the other side of the altar, or you may wish it to come from the altar candle. Picture it forming clockwise (deosil) all the way around you. You can visualise this as a white light forming a ring or as a circle of flame. Once the circle is complete, visualise the light extending slowly upwards from the circle to form a translucent dome or cone of light around and above you, completely enclosing you inside. It will feel like the inside of a bubble.
Take another minute or two to relax and feel your energy filling the circle. You might wish to visualise this as a coloured smoke to help you feel it better. When you are ready and relaxed, you can start you spell.