You purchase the downlander online, at least two days before travel.
When ordering, you have to state a means of identity that you will have with you on your travels.
When complete, the ticket appears on screen and you print it off.
Take with you, making sure you have the selected form of ID in your possession as well.
Had a number of these and they are excellent value.
If planning to use the buses in the Brighton/Eastbourne area I would also print a copy of the validity etc, as some bus crews have never heard of them.
Happy travels!