You decide to make yourself a woman, because no one would suspect someone like that to be at a Halloween party. Ceridwen watches you write down sentences, as you get a bit taller than normal, while you write down your own penis beginning to shrink, while you replace it with a small slit which was a vagina.
You also write down that your body hair was shedding, your hips were shrinking, your butt expanding, your face becomes maturely feminine, your hair gets longer, you grow big breasts, and your voice gets higher. All the changes you've written down happen in real life, as you feel your clothes getting tighter, your voice getting higher, and you getting older, as you were coming to almost the same age as Ceridwen. By the time the changes were over, you took a look at yourself.