How can I make sure that the words I write are my own?
I don't know what the purpose of your writing is, so the answer really depends on that. It sounds like you're writing some kind of review of prior work (either as a survey or as part of a paper) ? In that case, the goal here is not to regurgitate (in your words or via paraphrase) what others say. Rather, you should be reading what they say and thinking about it (and seeing if you're convinced by it). 

Only after that should you even attempt to describe the work. And when you do so, put all reference material away. If you can't describe someone else's work without referring to it, then you don't really understand it yet. In this way you'll ensure that you use your own ideas/thoughts to express what's gone before.