I pinned my cutie mark onto my shirt: lower-right, front-favoring. As long as it's visible, and looks cool, though, I think it's really up to you!
Also, for a tail, I started out using sort of wig-hair tucked in the back of my skirt, but found it really looked more creepy than cute. That may be in part, though, because my cosplay uses a hat with a plushie mane instead of a wig. :X Tails seem to be very forgivable, so if you don't have an idea or don't like the look, you're fine going without one. ^^