To earn your charms, make sure your account is set up to receive Twitch Drops, then subscribe to the participating streamers channel:
Charms will automatically be added to the in-game inventory of the Ubisoft account that is linked to your Twitch account.
These will appear within two hours of subscribing and you will keep them even if you unsubscribe from the channel.
If you stay subscribed to the channel, you will automatically receive the new charms at the start of each season. Otherwise, you will need to resubscribe or redeem a code received while watching a stream.
Each charm can be only claimed once per Twitch subscription.
You can access the charms across all Rainbow Six: Siege platforms, as long as they are linked to your Ubisoft account.