I don’t recall any instruction on it, and I was definitely creating stacks at age 8 or 9. I recall it being quite straightforward. I made a slide show presentation about subliminal messages for my 7th grade science class (1992 I think)

I admit we had a Mac early on, so I was very familiar with interactions by that point, but I found HyperCard being shockingly intuitive.

Every 7th grader in my middle school computer class learned to use Hypercard and I wouldn't say any of them were tech savvy at that age in 1990.