I think that it comes by a lot of writing, and that's why I like to keep the course writing-intensive. That kind of analysis of primary sources distinguishes the writing of history from other writing. Students themselves become practicing historians by reading primary sources and interpreting them, so they're constantly exploring what it is that a historian does to reconstruct the past. It's also part of the way they learn. Unless you write things down or say something out loud, you don't really know what you're learning.
Writing is part of what's going on in the classroom.
I have them write in the class and use their writing to form the basis for discussion. They're learning in the process of writing and they ARE able to express themselves better orally. They become much more verbal over the semester. I assume that no matter how good they are or how poor they are at writing, they can all benefit from starting at square one and breaking the writing task down to its stages. Some of the graduate students initially resent that because they can jump straight to analysis.