1. You will receive a voucher / key like this: 7899-DA67-2E92-XXXX

2. Log into your account ( If you haven't account at https://ffdownloader.com/, you just need to register )

3. Make sure you are logged in, then open page: https://ffdownloader.com/premium

4. Put your Premium key / Premium code in "Activate your premium key:" and click on "Activate"

activate ffdownloader premium account