Practice the historical-thinking skill of comparing multiple perspectives on the same event, using the mission system as the example, and asking whose voice survives in the record.
The same set of events can look completely different depending on who is telling the story. A Spanish priest might describe the missions as bringing faith and a new way of life to people he saw as needing saving. A Spanish soldier might describe the missions as a smart way to secure land for the king before rival countries claimed it. A Native Californian living at a mission might describe the loss of his village's freedom, land, language, and choice over his own daily life. All three people could be describing the exact same mission, honestly, from where they stood.
Historians ask an extra question here: whose version got written down? Most surviving written records from this era come from Spanish priests and officials, because they were the ones keeping journals and official reports. Native Californians' own accounts were passed down more often through oral history, remembered and retold by families and communities, rather than in the mission's paperwork. A careful historian looks for both kinds of sources and asks what might be missing from a record kept mostly by one side.
Mastery looks like: They can describe the same mission-era event from at least two different, realistic perspectives and explain why written records favor one side.
Common stumbles: Assuming there is only one "correct" perspective on a historical event; dismissing oral history as less real than written records.