The best way to unlock the new content.
Play and get rewards.
The Battle Pass will be available for 750 Rubies. This gives access to unlock all rewards. There is also a free path.
Progressing is done by Battle Pass XP. You will gain said XP by playing with cards with high Glory.
If you buy the Battle Pass at a later point, you will receive all of the rewards up to your current tier, retroactively.