When he's upset, I put a hand on his back (bare skin) and hold it there, or I hold him on my chest while he breathes and I breathe with him.
Or if he falls over and is further away from me, I look at him, take a few big breaths and suggest that he takes some breaths.
By 18 mo he was often doing the deep breaths automatically.
What I mean is, if someone showed me "deep breaths" I wouldn't conclude "people have emotions, they can affect us adversely, some times we don't control them if we're absent minded, better put some conscious effort into keeping them in check", least of all when I was 18 months.
I'm talking about the practiced/done skill of emotional regulation. Seeing his ability to self-soothe relative to other kids at 18 months, it's very clear that he has a solid amount of the skill of emotional regulation, even without any understanding of what it means.
He climbs up somewhere and gets kinda stuck. I go over to him and put my hand on his back and breathe and invite him to breathe. Then I spot him, but I don't help him down, I encourage him that he'll be able to get down.
Often, he only wanted his mom to do the breathing with him. That's fine. He falls, cries, we both offer him a hug, he goes to mom and she breathes with him while he cries and I put my hand on his back while she's holding him and offer him a gentle gaze if he wants to look at me, while I breathe with him as well.
I didn't do the "hand in skin" part, because he had his clothes on and when I tried putting my hand inside his clothes it just annoyed him more.
He was having a tantrum. I sat on the couch and sat him on my lap facing me. I pulled him towards me chest on chest. He quickly got more annoyed to be held down but I kept him there. I started doing deep breaths. It took him a bit to realize that we were "doing something" but slowly he started settling down and within 10 minutes he had stopped crying. This is in contrast with "normal" where if I hold him down while having a tantrum it just escalates. Eventually I stopped when I noticed that he was about to fall asleep.
It's unclear whether the deep breaths actually contributed or whether this was just a lucky one-off. I'll continue this.
I also got him to do a few times a full 5 minutes of yoga in a row jaja.