The second bullet of the ritual caster feat outlines the costs of the process, and those rules are unchanged for Adventurers' League. The AL Player's Guide (on page 3 in v9.1, current at the time of this post) talks about how many hours you can spend in each downtime day copying spells.
The first part of the second bullet is probably the part you're stumbling on: "If you come across a spell in written form".
If you've picked any class except Wizard, the only "written form" you're going to find is spell scrolls. Sometimes you'll find them as loot during adventures, but the selection is quite limited. Fortunately, the AL Player's Guide (also on page 3 of v9.1) provides prices for buying scrolls up to fifth level1 .
If you've picked Wizard, you can also copy spells from player character wizards. This uses the exact same rules as two wizards copying from each other.
1There are only two ritual spells above 5th level - one Wizard (Instant Summons) and one Cleric (Forbiddance).