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There's no need to include the "/r/" at the beginning of the name.

Once you've entered one or two subreddits, the page will suggest more subreddits under the "people also added" heading. Click the + icon next to a name to add it, or click the name itself to visit the subreddit in a new tab.

You should now see a blank subreddit. Look for the "add subreddit" text box on the right of your screen. Type in the name of any subreddit and press enter. Repeat this with any number of subreddit names. As you add each subreddit, you should see its posts appear on your multireddit feed. That's what multireddits do: combine posts from multiple subreddits into one feed.