From your 'My Hub' page (please see below)
On the left hand side of your account, you will see the side menu where you can select 'Packages'.
Then click 'Vouchers' from the 3 options at the top of your page.
You can then start creating your discount voucher by hitting the 'Create Voucher' button at the bottom of your screen
Fill in all the details in regards to how much discount the voucher will offer, only use letters and numbers for the code name.
Forever means the discount will applied on every payment of a recurring package and one time means it will only discount the first payment on a recurring package.
And finish by clicking the 'Create' button.