Mark or highlight the areas you didn't agree with in a novel. Ask yourself why you aren't happy with that chapter or scene. If it is because the two characters you wanted to get together went their separate ways, that's personal preference. Instead, focus on things such as the writer's voice, the flow of the writing, grammar and spelling.
Give constructive criticism whenever you can. Avoid using phrases such as "This part doesn't work for me." Explain why it didn't work for you and what part of that chapter or scene confused you. Help the author improve the manuscript by articulating what is unclear and disorganized. Good communication is especially necessary when working in a writing group.
Share something you liked about the story or the reason why you chose to read it. Explain how it didn't meet your expectations, and what improvements you think might make it better.
Give the writer resources on grammar and punctuation if you feel that is an area of weakness. You can find numerous writing resources online for free.
Include notes on world building whenever you can. A writer intimately knows the world of their creation, which may cause them to neglect the inclusion of a small, vital detail. Spot these plot holes and point them out to the author. Ask the author what details should be included to accurately portray the world they have built.
Do not expect the author to make all of the changes you suggest.