You confess such a sin just as you would any sin you need to confess. In this case you might say:
I dabbled in the occult for the last ten or so years and had recently completed a Wiccan study of “a year and a day,” enabling me to be a first degree priestess for one of my local covens, but stopped short of being initiated into the religion.
Your priest should give you some counsel along with your penance. I will only mention one consideration here.
Most people who have never indulged in actual occult activity have no problem reading books that contain fantasy witchcraft (e.g., Harry Potter) or watching movies with fantasy witchcraft (e.g., The Wizard of Oz). It may be that, for you and for others who have dabbled in actual witchcraft, such entertainment is a trigger for temptation to “backslide.” As Gregory K. Popcak explains in his book Beyond the Birds and the Bees:
There is a wonderful part of C. S. Lewis’s book The Magician’s Nephew in which a golden apple tree … is planted to keep the evil witch out of Narnia for a long, long time. While the tree is growing, though, the witch sneaks in, steals an apple, and gleefully eats it. The child heroine in the story is confused as to how this tree can really keep the witch out, since she didn’t seem to mind eating its fruits presently. The child puts her question to the great lion, Aslan.
[quote]"…Aslan," she said, “…there must be some mistake … [the Witch] can’t really mind the smell of those [magical, golden] apples … she ate one.”
“Child,” he replied, “that is why all the rest are now a horror to her. That is what happens to those who pluck and eat fruits at the wrong time and in the wrong way. The fruit is good, but they loathe it ever after.”
In other words, it may be necessary for you to avoid literature and entertainment that is otherwise okay in and of itself but represents a temptation for you. Please be careful though that you do not confuse the need for necessary caution on your part with a blanket restriction on others who do not face those same temptations.
God bless, and welcome home!
Witchcraft 101 by Michelle Arnold (sidebar**)