So, after I was asked about why we put the sigil on the mirror, I thought of something. I can't believe I hadn't thought of it before, and leaving this caution out is very unwise of me:
I mention that you could use the rough method of treatment for thoughtforms and the mirror for standard conjurations. What I failed to mention is the most obvious thing, and makes me feel like an ass for forgetting to say... If you do a standard evocation with the mirror, take standard precautions. The spirit will be much more capable of obsessing you than a thoughtform ought to be. In fact, if you find yourself becoming convinced that you are Batman, or Superman, or generally obsessing [the character] following the ritual this is a sign that you need to take more time to focus on your basics: meditation, developing the will, making sound protections when you work. It should not be necessary, as you should have been doing all that long before considering a full scale working of even the most basic sort.
Finally, if you do a standard conjuration of a spirit from any magical book, it is [probably very] unwise to use both your reflection and the hypnotic state to see the spirit, and placing the seal of that spirit on the mirror. That is a probable recipe for obsession for the unprepared. It is even very unwise for scrying up visions of Gods. You don't want to get obsessed with Pan. Trust me on this.
If what I'm talking about doesn't make sense? I suggest doing more reading on the subject before considering doing anything with the mirror and spirits.
That said? The sigil can be used, never being superimposed over your face, and preferably above the area of your reflection, to designate areas you wish to see. (Note: I have never had much luck with scrying physical locations. If you end up being good at it, tell me what the hell I'm doing wrong. I'd love to know. Astral locations have worked much better, for me, overall.) As per usual, your mileage may vary.
Be careful, even if it is a beginner's ritual.
I have mentioned and recommended Volantia prior to mirror work. I would double this and add that you should seriously consider using Paschal's other techniques, too. They are meant to go along with the work, and they do suffice to help.
Also: Rachel is right. For those wishing a beginner's technique not involving Chaos-y things, a basic ancestor altar will be an excellent place to begin. But, I think she should make that post if she hasn't already.
PS. Actually, now that I think about it? Don't ever, ever use that ritual style at all for a standard evocation. You want all your crap on hand for that: wand, pentacle, seal, etc. I would edit out my asshole swaggery, but I'll let it stand to show what I fool I am. I have edited in an alteration for any future readers. Jesus, I'm very sorry. What the fuck is my problem, man?