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This is found on the upper-left corner of the screen. You'll be shown all the featured and available channels. This will enable you to view the channel's profile. On the channel's profile page, you can tap "Preview" to read sample posts. Once you've chosen a channel, simply tap on Join, found on the bottom center of the screen. The channel will now be listed in your Subscribed Channels section.
Tap on the Channels icon. Tap on a channel. Tap Join.