The first step is to think about going back to square one. Pretend you have never touched a sewing machine before. Most people will have an older sewing machine that they want to learn to sew on rather than make a big investment. If you don't know their machine, ask for time to test their machine so you know it is in working order before the owner/student gets frustrated trying to sew with a broken sewing machine. If the machine is not working correctly, tell them to get it repaired before they attempt to sew with the machine. Have the machine owner obtain a manual for the machine if they do not have one.
Learn the parts of a sewing machine so you can correctly explain how things work and use the correct name of the parts while you are giving directions. Saying the "thingamajig or the whatchamacallit" is not the way to teach someone because they will never find those terms in a book or manual when you aren't available.
Set up a supply list with the materials you will expect the student to have with them. This can depend on a particular project or a complete sewing kit and sewing machine. This will be a decision you will have to make. Their own sewing machine, bobbins and thread are the bare minimum. A student is much better off learning on their own sewing machine when ever possible. Try not to overwhelm the student by asking them to have sewing tools they will not need immediately but ask them to have enough tools that they will be able to practice and sew on their own when they are not with you. Part of their preparation for sewing lessons should be to sign up for sales fliers and coupons so they can save money by sewing. You may also want to suggest that the student have a folder or notebook to keep their own notes or scrapbook type pages of their progress.