"There are no hard an fast rules, unless you want to tie yourself to a particular persons approach, in which case do what they say. There are some pretty specific ideas in older grimoires, but they're not thelemic. Book 4 has a chapter on the magical circle as well as one suggested version based on an older version but "Thelema-ised". http://aleister-crowley-books.com/im...c%20Circle.jpg (link dead)
The basic principle is you need a circle. That's it. Some folks don't even require that, as they see the circle as a physical representation of the circle created energetically by your banishing or in your aura by greater banishings. If your more comfortable with a physical circle, you can "fortify" it with holy names of your choosing (e.g. In the image) and suitable colors.
Here is a classic from the Key of Solomon,
http://www.sacred-texts.com/grim/lks/img/fig153-4.jpg and if you have Duquette's autobiography it contains one featuring some Coleridge poetry as the inscription."
And from another person: "You can create a circle with a pack of Tarot cards."
So I did the latter with my new Rider Waite Tarot pack I bought today for today's daily ritual (normally I just banish). I quite liked it! I also have done a reading already being exhausted from lack of sleep and loads of theoretical training at my new job I wanted to get out of an appointment, I was just gonna ignore I had it (as really tired people who cba can do) but I decided to shuffle my deck for the first time and draw a card: Seven of Swords - http://www.learntarot.com/s7.htm, I was e-mailing my friend at the time and she said her explanation of the cards said " "this card emphasises the need for diplomacy to attan your objective, do not use aggressive tactics. Use your brain to achieve your objective." (the book I got had even another text that was very fitting to the situation but I'm too tired to copy it and explain the situation more.) So I slapped myself, contacted the people politely explaining my situation and got excused, yay! Do we really need tarot for that? Well when we're this tired it can help to let cards straighten you out when your brain is asleep ;).
I don't think I ever had a reading that didn't make sense (I usually rarely do a tarot reading too but gimme a break I bought them today ;p). I might buy Douglas Rushkoff's Cyber Tarot program (if it drops in price and is compatible with modern computers :p) too, it's available on Amazon now as opposed to being free on his website, but that program was very nifty (and had a wonderful modern deck with Carl Jung etc.), will give my new cards a good run first though, can't wait to experiment on the Kabbalah's Tree of Life Paths with them.
Update: I wanted to draw one card to inform me about the knowledge and conversation of the holy guardian angel, guess which card I drew, Seven of Swords, again, what are the odds lol.