Which competition are you referring to? I assume since you are posting to this thread then its for Dog Breed? The code can vary quite a bit depending on which one it is 
Generally speaking I always start by using the submission_file.csv that kaggle provides for each competition as my df with a simple line like submission = pd.read_csv('sample_submission.csv'). From there you just replace the ids in the sub with your test ids (so they are sorted correctly/aligned with predictions) which you can grab from here data.test_dl.dataset.fnames. Then you fill in the rest of the columns in the df with your predictions.
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