Don't be afraid to ask department heads questions about the material you are asked to present to students either to refresh your own memory or to garner examples on how to perform certain math problems in case you, like me, need clarification of the regular teacher's plans at the high school level. You are showing your intelligence rather than any weakness. I am not pretending to teach a subject that I do not know. I am only checking my memory or looking for examples completed by those who know how to perform such mathematical operations so as to assist the high school level students in the subject matter where I am not the expert. The substitute service knows where I am qualified, English, literature, and writing; however, sometimes they really need any substitute to assist the students on a particular day.
I've devised plans, though, on how to cope in such a situation, if there is no time to see the department head or a fellow math teacher before class. I always walk around the classroom checking that the students are actually doing the assignment. I inform the students that if they have a particular question, they should jot it down on their papers so that the teacher may address it when he or she returns. But I also find out who knows how to perform the operations and therefore the problems as most times I am presenting students with review work in the upper levels of education, and I ask those students to kindly assist their fellow students if necessary. Most students enjoy helping others, and I always learn something in the process. I keep notes so that if I am faced with the same subject matter again, I am prepared...somewhat.