Do not write from your sources. Write from your notes or from your memory.
If you’re following my rules, you are always writing about something you decided yesterday that you knew last week. You should pick something that you not only think is true but understand why is true. It should require no additional research on your part to be in a position to assert it in writing–to make a claim and support, elaborate or defend it in a paragraph. All of these issues have been settled the day before, during five or ten minutes of careful reflection.
You don’t have to have the entire basis of your argument “in your head”. Knowing something does not mean memorizing every important fact about it. But you should be able to quickly and easily locate the relevant information in your notes. After formulating your key sentence, you should be able to reach for the set of notes in which the sources of your knowledge are clearly stated. This should take a few minutes at most. If it’s going to take you an hour or an afternoon to find the sources of a claim then you don’t know it well enough to write about it. Choose something else for tomorrow and resolve to understand the other thing better.
Imagine you are giving a lecture and you assert some matter of fact. A member of the audience asks you to back it up with a source. You say, “I don’t know it off the top of my head, but I can get it for you if you send me an email.” By this you don’t mean you’ll need a few weeks to discover whether or not what you were saying is actually true. What you mean is that back in your office you have easy access to the source. Maybe you’ve got some notes, indexed in a useful way. Maybe you know exactly what to type into Google to bring it up on the first page of search results. That’s fine. It’s doesn’t matter what kind of access to the source you have; it just has to be quick.
Some people don’t take very good notes but have a great memory. They might say they know roughly where in what book the information they need is to be found. Very well, as they are making their decision about what to write tomorrow, they should find that book, open it to the relevant page, confirm the accuracy of their memory, and then put a bookmark in. Now they just have to bring that book to their writing session. The danger, however, is that they will take too much of the style of their source with them. In the worst case, they inadvertently plagiarize their source. So I would recommend making a quick note of the key facts and (clearly marked!) quotes, including their page numbers, and then just bringing these notes to the writing session.
The other danger is that you will start reading when they should be writing. This is also why I recommend simply putting your source texts well out of reach as you sit there and write. You don’t need a library to compose at least 6 sentences and at most 200 words in 27 minutes. You just need a few jotted remarks to remind you facts, figures and names. As the brilliant poet Ben Lerner once said (though he was probably thinking of something else altogether):