In the past (and also these days), I have added written instructions to the payment selection area on the checkout page.
Generally, these are coloured RED and in BOLD text - so the idiots who do online shopping (that's me too), are instructed what to do before hit "Confirm Order".
EG: Above the option for CARD PAYMENTS I have;

Make sure to enter your card details manually! All fields are needed - CARD NUMBER, EXPIRY DATE, CVV