OPENING THE CIRCLE
When opening a circle, make sure you/everyone is inside of it first. Everything you need should be inside, too, as you don't want to break it. Sweep the inside with a broom, splash holy water inside, or smudge the air within the circle to purify it. The outline of your circle can be anything at all: drawn from chalk; powder poured in a circle; tarot cards displayed in a ring. I prefer to change my circles depending on the spell (for a love or friendship spell, I like to use rose petals, and for protection spells I like to place nails in a circle). If you draw the circle from the outside, you may place a broomstick at one side to act as a doorway, though not all do this. This doorway will allow you to enter and leave your circle without disrupting the energy too much. Light the candles/greet the elements starting in the east and going deosil (clockwise).
It sounds silly, but you should think about clean up before you even cast a circle. I have candle holders and mats to form my circle, but other people prefer to have candles on the bare ground or to mix potions without anything to catch any spillage. My advice is to always keep a few pretty tea towels or cloths within your circle, in reach for if you spill anything. Also, I feel it's best to have candles on something, especially if you're inside: it can be a nightmare getting wax out of carpet so try using candle holders or small plates.
When I was first casting circles, I was always forgetting things or placing things out of reach, meaning I would have to break the circle, therefore weaken the spell I was casting, jut to retrieve what I wanted. Make sure you have all you need in your circle space before you cast it: try making a list and ticking things off before you light any candle, just to make sure. Along with the tea towels I advised for clean up, you might want to keep a small box of spare bits you may need in case something goes wrong, such as spare candles, matches, or even hair ties for those with long hair (you don't want to end up burning your hair in a candle flame).
Are you inside, outside? What's the weather like? What time of day is it? How much time do you have? Is it quiet? Are you likely to be disturbed? Think of whether or not it's appropriate for you to cast a circle at that moment; getting up to answer the door or having to listen to noisy neighbours can be irritating and disrupt your practice. Plan the right place and time for your circle to be cast and be realistic. If you're outside in a storm, it's unlikely you'll be able to light candles or do your circle without being distracted, and even if your kids/housemates have gone to bed, don't be surprised if one of them suddenly disturbs you in the middle of the night. If you're casting a circle inside and you live with others, try posting a sign on the door saying for you to not be disturbed.
If it's just you casting a circle, your organisation is simple: have the right supplies. But, if you're casting a circle with others, your organisation may be squiffy. People can end up tripping over one another, so make sure your circle is big enough to fit everyone inside. Plan who says and does what, and where people should be: if everybody's crowding around, waiting for orders to do something, there will be a lack of focus and organisation. Make sure you organise the people and your supplies.
CLOSING THE CIRCLE
NEVER blow out your candles. It can undo all you've tried to do. Snuff out the flames with a candle snuff or damp fingers (if you can't do either, just place a cup over the candle and it will burn up all the oxygen inside, causing it to go out). Start by snuffing out the last candle you lit, which could be the north candle or any others you have inside the circle. Snuff out the candles at each quarter in a widdershins (ant-clockwise) direction. I wouldn't use the candles again, unless you're specifically told to do so or if you were just honouring the goddess and god: candles used for spells should be buried or used just for decoration as the energy left over from the spell inside them can have an effect on other spells you do.
-When I cast circles, I prepare not just the space but myself: I wash with organic lotions, rub in oils and do my hair up in a neat, but pretty, do, which helps since it's very long and I don't like it getting in the way. I also wear special clothes; I have a dress saved specially for circle casting. Try saving certain clothes or pieces of jewellery for circle casting, then you'll have something to help you focus more, such as wearing a uniform in school makes you feel more part of the community, wearing certain clothes/jewellery can make you feel more part of the circle.
-Meditate before and after circle casting so your energies are focused towards the spell/ritual to focus the universal powers more on your goals.
-If others are joining, let them participate, so their energies are mingled with yours and the spell can become more powerful.
-Be careful. Yes, they're just candles, but they still burn. Hot wax is a real nightmare if you spill it on you or your favourite clothes. Also, if an athame is being used, try not to get cut. It will not only distract you but it's a cut: it could become infected and all wounds should be treated immediately.
LASTLY circle casting is different for everyone. If you have your own methods, do them; these are just tips, not instructions. Do what feels comfortable for YOU.
Stay safe and happy casting.