Can you define tantrum for me? It's kind of a vague word. Learning is frustrating and it isn't a bad thing to be frustrated. We just have to work through it. So, maybe helping her. Often they won't take help directly as they want to do it for themselves but empathy followed by help works well. Acknowledge her feelings. So you can say, "Oh, that is soo frustrating!" but say it in the same tone that she is using. "That makes you so mad!!!" If she has stomped her feet - you stop your feet. In sales they call this building rapport.
But then say in your normal mommy voice, "But if we do it like this, it will be easier. See...Now you try." Just keep this up or change the activity if she is just not getting it.
You can hear all the kids in my house say, "Oh gracious, that's frustrating!" My little girl with Down Syndrome couldn't say gracious so she would say "Oh, Gracie!" Which melts my heart. That is a cue for someone to come assist. Or just observe the situation. You don't need to teach her to challenge herself. She is doing it all the time which is why she is frustrated. She isn't capable of doing all that she wants so she gets mad. But she is doing a lot. The goal here is to help her understand (eventually - not now, she is still little) that we all have to work at things. Start by never telling her she is smart, but by praising non-stop the effort and work ethic she has. How she continues when it is frustrating and hard. Hard work is a blessing, She is a hard worker. Good work should come freely from your lips. And reminding her when she accomplishes something that it was hard work. "Wow, look at that! That was hard work! Good Job!" Remind her how it used to be. "Remember when you couldn't even....Now look at you....!!!"