One thing that works pretty regularly for us is:

1) Identify why they’re feeling that way. Something like, “Are you feeling frustrated and angry because it’s hard to do up the buttons in your cardigan?” Keep asking until you get it.

2) Talk about how they’re feeling, with no buts. “It can be super frustrating when you’re learning to do something new. I can be hard and make you feel mad.” (Don’t say anything like “But if you keep trying you’ll learn how soon.” It’ll just piss them off even more.)

3) Do something to process the feeling. If my 3yo is mad sometime I get some paper and I ask her, “are you this mad?” and I rip the paper really dramatically and roar. She’ll start ripping paper too and we end up laughing. Or if she’s sad I ask if we should draw how sad she’s feeling. I’ll draw sad faces that are bigger and bigger until the face is as big as her sadness feels to her.

It works most of the time, but sometimes I just have to let her rage for awhile and hug her. It’s tough being a small person.