I tend to copy sections that I especially loved, and say why I loved them underneath.
I generally type up comments that way and then copy-paste to the comment box.

But the best way for me is to read a chapter and be so excited about it I just have to comment then and there. And I don’t think authors care if you’re a bit sqee-ful and say things like OMG I loved this, etc.

And/or I say what I loved about a particular character and what they did/said etc.