Under some circumstances, “they” can refer back to non-plural subjects:
“If the character writing the incantation fumbles their roll, their error will call the spirit of dead Cthulhu to life.”
“If a non-player character is killed by the vampire, they will return to life
Note that some English teachers will tell you this is poor English. They’re wrong. This gets into one of my pet peeves, which is trying to categorize the rules of a language, discovering that your rules are wrong, and then claiming that the people speaking the language are really the ones at fault.
In my experience, “they” can be used to refer back to non-specific nouns, especially nouns that, while singular, refer to a class of people. I would never use “they” to refer back to a specific individual. That may be because we know that specific individual’s gender, or, if we don’t, that in itself is important.
Why Write to All Readers?
It almost seems like a silly question: why do you want to speak to your readers? Obviously, you want to write your instructions so that your readers know that those instructions apply to them.