Similar to your outside knowledge, you must show contextualization.
In other words, you must connect the documents to the larger picture of events, processes, and developments occurring before, during, or after.
The more analysis, explanation, and linkage between evidence and thesis, the more likely you are to succeed. You should have a deep and complex understanding of the history and skill in the prompt.
See the DBQ information for more on Analysis and Reasoning, and Complex Understanding.
If its on Japanese imperialism, link it to the Meiji Era and modernization of Japan.
But like above, be in-depth, as a phrase just wont cut it.
The Long Essay is what you might consider a standard essay.
You will be given a question, and might have to consider how history changed, or remained the same over time.
Or, perhaps its an essay targeting the skill of comparison, or causation.
Nonetheless, this essay will need a thesis and historical evidence to support that thesis.