You will be reading extensively before you start writing your dissertation. Your professor or supervisor will tell you this right in the beginning that the content for your dissertation has to be culled out from extensive reading of existing literature that you do. When you read a passage or content and wish to incorporate it in your assignment/thesis/dissertation/project you need to first understand the essence of the document as a whole before you start making notes about specific phrases or ideas.
To begin with, read the text thoroughly, not once but several times. After that look away from the source and try to remember what you have read. Pen those ideas in your own words. if at any point, you feel that you are not able to achieves success in this, the only reason can be that you haven’t understood the idea well enough to articulate it in your thoughts. Have you also experienced something like this? Try to first explain the content to your study partner or friend verbally. It will help you to increase your understanding before you start writing the content.
If you are one of those students whose first language is not English, it is ok to first paraphrase the content in your own language and then in the second stage translate it into English. This will bring out the ideas more precisely because of your comfort level with this language.
Another technique for paraphrasing can be to break down complex sentences into simpler ones, or if the original content has short simple sentences, combine them to create complex sentences without changing the meaning of the sentences. in this process you will automatically eliminate some words or change them to alternative words or phrases. You can also take the help of dictionary or thesaurus to know of other word options. The important thing to be noted here is that, when searching for alternative words, make sure the precise meaning of the word does not change. Choose simple english words, this is more true for academic content where using specific terminology is important and becomes sufficient evidence for understanding the concept of your writing.
As a writer, you do have the freedom to use the content as it is, at few places where you feel the need to quote or changing the language or words may alter the meaning or significance of the content. In such a case you need to enclose it with “quotation marks”. make sure you don’t do this often, as the originality your writing will be lost and you may get caught in the trap of plagiarism, which is another tough battle to fight for writers.