What’s keeping anybody from doing it? A secret satanist could baptize a bunch of people, register it and call it good. Cool. I guess if authority doesn’t matter then why have a pope or priests or any hierarchy for that matter. Make it a free for all. Let everyone make up their own versions of baptism. Groovy.
In order to be valid, "their own versions of baptism" would have to include the words "N., I baptize you in the Name of the Father, and of the Son, and of the Holy Spirit" (or something very close to it), and would have to involve the administration of (something a reasonable person would call) water, probably either by immersion or pouring over the head.
As well as having the correct ministerial intent (see above), etc.