Plagiarism is putting someone work us yourself with out acknowledgement or with out citation. So, you must put proper citation if you are taking someone's idea.
For using only very close relevant of topics under simple categorization. The information should be nutshell - may be result or recommendations sometimes tools / methods . Never use more paragraphs - this is others study and their findings -should use genuine way . The importance is write and discuss about your study findings more informative and interesting way in your own words to reduce the plagiarism!
Inorder to avoid plagiarism; one must read other literature well and get the understanding. Afterwards, write in your own words bringing out your findings/outcomes and one must duely acknowledge the source as well.
This is a very important discussion because plagiarism affects the quality, objectivity and trustworthiness of your research. It ruins your integrity as a writer. Secondly, plagiarism is a criminal offence since it is about using someone else's work as your own i.e. stealing intellectual property. My advice is good interpretation, good scientific writing skills, good reference technique and the use of Turnitin, an internet-based computer programme to detect plagiarism.
Being in final stage of writing dissertation, what i do while paraphrasing someone else's writing. First of all i change the structure of the sentences and then change the synonyms.