Just to give you an idea of why "What would a baby step be?" isn't really a question we can answer, here's a FEW of ONE person's "steps" on their 14+ year journey to poly:
* Flirting and dancing with other people is ok.
* Kissing other people is ok.
* Fooling around with other people when SO can't be around is ok.
* Fooling around with other people when SO is around is ok.
* (lots of other steps in between)
* being in a committed and sexual relationship with someone I love other than my spouse is ok.
So, a baby step for you might be going on a date with someone who is not your SO (I'm assuming you are currently in a relationship when you talk about making the "leap" from mono to poly) with the understanding that physical contact on this date will be limited to what would be acceptable in public. Or maybe that's too much. Or maybe it's too restrictive. There's no way for us to tell what you and your SO feel comfortable with at this point, and that's the definition of a baby step: something different from what you do now that you're fairly confident you'll be able to manage. And then make sure everyone involved actually IS ok with the reality (versus the hypothetical) before moving on to the next baby step.
Pan Female, Hinge in a V between my mono (straight) husband, Monochrome and my poly (pan) partner, ThatGuyInBlack