Instructions can be seen here: https://www.flysas.com/en/help-and-contact/faq/giftcard-and-vouchers/how-do-i-pay-with-a-sas-gift-card-or-sas-voucher/
Just choose "I have a SAS Gift Card/Voucher" as payment on SAS’ website. Enter the code on your gift card/voucher. You can use your gift card/voucher as payment for the entire trip, or part of it, as well as travel extras. You can also combine payment with a gift card/voucher and a credit card.
You can use up to two vouchers at a time. When you pay with a gift card/voucher, any remaining value will remain on the gift card/voucher.