Great question Steph, I would recommend broadly a nice image with “Singing Lessons Southampton” as the main header “Singing Lessons” being the most important of that so the person knows what the voucher is for, your name, email address and phone number on. Teachers also sometimes put T&Cs on the back.
I would recommend doing one lesson vouchers each, so when someone is given 5 lessons they get five vouchers (looks like more than a single “five lesson” voucher). If you need any help designing the vouchers don’t hesitate to let me know, some examples below of what a voucher can look like: