I essentially, honest review or review swap, pretty much follow the same format for reviewing: pick a couple of things that I liked about the novel or that I thought the author did good in, and then list a couple of things that I thought the author could work on, and if I feel like it, list some additional tips as well. It's called constructive criticism, not destructive criticism, after all. Also I make sure that the tone with which I write the review is generally encouraging and on the positive side, because something a bit too negative could discourage an author from writing. Also keep in mind: is it something that the author can fix in a moment or over a long period of time? If you didn't like how the plot turned out or an aspect of a character, who knows, maybe the author intended it to be that way. But grammatical mistakes, on the other hand, are something else. Author's do appreciate corrections, and constructive criticism encourages maturity.
And please, even if you think a work is utter trash, never say that. That is something that the author has worked hard on. Just say something along the lines of "could use improvement". There is a delicate balance to find, an art to reviewing. And be mindful of how your words might come off as. Don't be overbearing over things like plot, character, and theme, because the author might have a specific vision in mind. It is their story, after all.
TL;DR: Constructive criticism, not destructive criticism. Give compliments as well as critique. Keep in mind that this is the author's story, and that they have feelings, but also remember that author appreciate feedback and honesty.
