Start by signing into your Store account.

In the iTunes Source list, select the playlist you want to publish. (If it contains any songs that Apple doesn’t sell, or copy-protected songs you bought elsewhere, they’ll get knocked off the list—which may ruin your carefully constructed mix.)

Click the arrow next to the playlist and click the Create iMix button in the box that pops up. You now find yourself in the iTunes Store.

Click the Publish button after you fill in all the info about your list.