You're right, the associated operating instructions are completely useless. There is also no link to instructions on the internet! However, I entered the search term "Xiaomi Smart Band 8" on youtube and found more than one useful tutorial there...

unfortunately there is nothing clever about it. You can find out most things with a little trial and error. Sorry not to be able to say more.

Maybe a little better:
https://www.bedienungsanleitung24.de/xiaomi-sma...