My iPhone is backing up to my iPad. How do I get it to back up to my phone only?

iPhone does not back up to iPad, nor does iPad back up to iPhone.

Either can back up to a local Mac or Windows system, or to iCloud.

iPhone and iPad can share some information via iCloud when that’s configured and enabled, such as sharing contacts information. This might look like a backup from one to the other, but is not.

No idea what you're talking about. Not only can you not back up your phone to an iPad, you certainly can't back it up to itself.

Writing an effective Apple Support Communities question

You can’t backup to an iPad or an iPhone. You can backup to an iCloud account or to your computer. What are you trying to do and what do you see?