Use Specific Release Verbiage

First, you must use this release verbiage (or something similar) on your contest posts:

Per Instagram rules, this promotion is in no way sponsored, administered, or associated with Instagram, Inc. By entering, entrants confirm that they are 13 plus years of age, release Instagram of responsibility, and agree to Instagram’s Terms of Use.

You can find the terms for Instagram contests here.

Don’t Ask Users to Tag People in Photos They’re Not in

Second, you can’t ask users to tag themselves or others in a photo in which they don’t appear. And you can’t tag people in a photo in which they don’t appear. However, you can tag them in a comment so saying, “Tag a friend below” is perfectly okay. They just can’t be physically tagged in the photo or video itself.