The charring solution works because of the tendency of ammonium chloride to decompose, when heated, to give ammonia gas and hydrochloric acid. The ammonia gas evaporates into the atmosphere, leaving behind strong hydrochloric acid, which is what actually chars the page. This sounds nastier than it is, in practice. Prudence dictates erring on the side of caution and always working with strong ventilation, but, frankly, I did this in my closed kitchen with the air conditioner on and didn't notice much more than a slight burning smell.
An easy way to prepare a saturated solution of ammonium chloride (or any salt, really), is to dump a bunch of it into a jar and add water, but not enough to dissolve it all. So you end up with a layer of solid at the bottom of the jar, all the time. The liquid layer on top will always be saturated with the salt no matter what the ambient temperature may be.
I'm totally thrilled with my results, but if I were to do it again, I might try a slightly heavier laser printer paper, perhaps some of that resumé-grade stuff, just to see if it ended up feeling more like antique parchment at the end of the process.