Guide to reviewing a scientific article
I suggest you look at the other reviews where your review will appear. Then you can write yours using a similar style and level of technicality.

You can also ask for guidance from the person who asked you to write the review. That might be best done by offering particular paragraphs as examples of what you might say. 

I doubt that there is a "guide" anywhere to help you with this, other than general guides to good writing.