Hmmm well, if you believe in magick you probably believe in the paranormal, the spiritual, right? Try starting up a conversation around those topics that they might be more comfortable with, see how they feel, and if you think it's safe to tell them you could try to segway into it. I guess witches can fall under "paranormal," AND since it's so close to Halloween/Samhain you can just say you're really excited for the holiday if they start to give you negative vibes about what you're saying.
In my experience, I've always just simply stated that I believe in magick among other less conventional things, and left it at that. If you just jump into "I practice magick!" out of the blue, depending on your other interests and the openness of your friends, you'll just freak them out.
On the Halloween track, you could always print out an article on real witchcraft and how it isn't "evil," to show your friends during a sleepover, or just for the heck of it, like "Wow, I didn't know such-and-such, it's kind of interesting, huh?" You don't have to push the topic on them, but I understand wanting to know who you're safe around in case you do come out of the broom closet in the future.