How do I move loose tracks into an album? 

Head to your discography page, view the track you want to move, and click the Edit button beneath the title.
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In the track editor, check the "part of an album, EP, what have you..." drop down, select the album that you want to move it into (please note: the album must be published). This will move the existing track into the new album, it won’t duplicate the track.
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When you’ve selected the right album, don’t forget to click "Add to Album"! Voilà.