In the early fifteenth century it meant: something that is true, or a "solid assurance."
In the the seventeenth century some quakers came along and declared it as "oath-taking."
In the twenty-first century witchy astrologists* reclaimed it.
Here's how it works:
On the day of the New Moon, find a sheet of paper. Date it and write the following: I accept these things into my life now for the highest good and for the highest good of all concerned.
This is called a karma free clause. It's your new BFF.
Below your karma free clause you're gonna write down all the things you want to manifest in your life. Don't waste time wanting or needing things. Focus on the "you," you want to be. All of your sentences will start with "I am." You're taking an oath, so no nonsense allowed.
Go nuts. Dig deep. Write a novel if you have to. Just keep in mind you're going to take this piece of paper and put it in your bra, underwear, romper—whatever it is you wear these days—and carry it on your person 'till the full moon strikes. It might try to escape, cause that's what dreams and aspirations have a tendency to do. Be strong. don't let the "real you" get away.
On the day of the Full Moon you'll burn this little sheet of magic in an urn of your choosing. I always end up using a bowl from my kitchen, but if this is the first time you're tending to your true self, buy yourself a special dish. I hear Anthropologie has some to spare.