There are a few 'musts' of ritual and a few 'optionals' and quite often Witches will disagree on this! Below is what I think, but feel free to experiment.
1. Plan the ritual. It's a good idea to think through what you actually want to do, what you want to use and what you want to say. This means that you can assemble everything you need beforehand so that you don't have to leave the circle. Make a list, including things like matches!
2. Cleanse yourself. Either take a bath or shower and relax and cleanse yourself by letting go of worries and concentrating on what you are going to do. If you cannot do this, wash your hands and face, preferably with something that smells appropriate. You can buy some very nice Wiccan products, especially developed for ritual from Wicce.co.uk.
3. Create Sacred Space by cleansing the area. Use my method, if you wish. This is usually a must but can be omitted when working in ancient woodland, where the place doesn't allow (a quick spell in the street, for example) or if you have a special room set aside.
4. Cast the circle. Use my method, if you wish. This is usually a must, but could be omitted when doing a quick spell in the street. Although, how long does it take to set up a personal circle using your imagination?
5. Call the Quarters. I do not and don't feel it's necessary.
6. Call the Goddess. Call her in any way you wish and use candles to show her presence.
7. Meditate/make the spell or ritual. By this I mean that if you have decided to make something in the circle, do it now. Otherwise, meditate on what you want to achieve.
8. Raise power. If you do not know how to do this, try my method. Cast the spell/release your intent.
9. Ground yourself by having Cakes and Ale or by letting excess energy flow into the earth. Give thanks.
10. Say Goodbye to the Goddess and extinguish her candles if you have lit them.
11. Close the circle.
12. Give your libations to the earth, if you have not already done so. Tidy up anything that needs to be finished for the spell/ritual. Tidy up your altar.