Click the three dot icon next to a track you'd like to add.

When you do that, the track's page will appear where you'll see the "Add to Crate" button.

Click the "Add to Crate" button, and a new page will appear where you can create a new crate or add a song to an existing crate. Click the plus sign icon next to "Create new crate"

A pop-up will appear where you can name your new crate. After typing in your crate's name, click the "Create" button.

Once you do that, your new crate will appear on the page. Check the box next to the crate you'd like to add the song to, then click the "Done" button.

Back on the track's page, you can click "Close."

You can locate your new crate using the "Crates" button at the bottom of the screen. After clicking that, select "My crates" at the top of the page.