It's very useful to gather (undisputed) facts and contextual information before you start. Scholarly encyclopedias or other books can provide useful information such as:
- Undisputed facts of an event (date, location, events, etc.)
- Controversies or disagreements about the event
- Information about the context of a historical event, and the impact that event had on the world
- Names and roles of the key people involved in the event, including active players or stakeholders, or those impacted by the decisions of others.
- Bibliography, or 'further reading' to learn more
- Notes on the historiography of the event - how historians viewed that event and how that changes over time
- Key primary sources - documents from the event, or references to memoirs or other later writings
All of this information will help you to find, select, and evaluate the primary sources that you will find.