Writing theory section of thesis - feels like I am just copying I am doing a PhD in physics / applied math and have just started writing the theory section. A large-ish part (5-10 pages, estimated) will be setting the scene with some theoretical tools that I am using but did not invent. These tools will eventually be used as the basis for analysing data.

I will say the word so you don't have to think it: plagiarism. I have absolutely no desire to plagiarize so am very careful about citing my sources - however while writing I get the feeling that I am simply copying off of others.

Question, in two parts: 


Can anyone give a strategy for how to write a theory section and make it ``your own'' while clearly citing relevant material? 
Is this feeling actually normal and am I just over-reacting?
Although I'm not in "physics/applied-math", but, rather, "math", I see the same or similar issue arise. Let's consider the point that, by this year, the "context/background/set-up" has very likely been highly optimized, both for effectiveness and for succinctness. Nothing wasted, nothing superfluous, etc. So it is likely to be hard to "improvise" much on its description without making it worse in some way. This may be the case even down to small notational conventions, wording, etc.

So, be absolutely scrupulous in citing, and try to have it all in your own head so that you can write it yourself (not literally copying), even while citing, ... and remarking something like "we recall some standard facts/ideas/set-up, e.g., from [X]."