Had better do

Thank you first for your attention to this question.

The thing goes like this:

I was taught that this phrase had better do was used to ask somebody to do something, and it was good to do so.

Later I learnt that this phrase was kind of strong, and it suggested something like warning.

So my question is when I can use this phrase?

For example, if I just want someone to something instead of doing otherwise, nothing serious, just something small, trivial, can I use had better do...?

And, if I were a customer service, could I use you'd better do... when I explain to a customer how to use a product? Suppose that the thing I explain to the customer is not dangerous and it won't cause life and property damage. And how about it is dangerous?

By the way, are those words marked red grammatically correct?

Thanks in advance. I'm sorry this is long and maybe complicated.