For an existing recipe, go to your user account page and click on the recipe you'd like to submit. Click the "Edit" button, scroll down to the bottom and select the option that allows you to submit the recipe to a contest. Click "Publish This Recipe" to finish submitting the recipe to the contest.
For a new recipe, upload as normal, and find the same drop-down menu next to "Submit My Recipe to a Contest" in the edit screen. Hit "Publish This Recipe," and you're done!