Boolean operators are words you can use between search terms to either broaden or limit your search.
Use OR to broaden your search. For example, if you search advertising OR commercials, you will find articles that include both terms.
In contrast, use AND and NOT to narrow your search. For example, if you search advertising AND commercials, you will only find articles that include both terms.
And, if you search advertising NOT commercials, you will find articles that include the word advertising except for those that also include the term commercials. In Google, use a minus sign instead of NOT (e.g. advertising -commercials).
Think of a relevant topic that interests you, and look for key concepts. Take those concepts and brainstorm related terms. Since you are searching for secondary sources that comment on a primary source, the name of your source may be one of the terms.
1) identify a question or topic
2) Look for key concepts
(see example below)
How does An Old Woman And Her Recollections dictated by Doña Eulalia Pérez to D. Thomas Savage, reflect power structures present in early California Missions?
You might begin your search by looking for articles that discuss life in the Missions. You might also develop keywords pulled from the source itself, or related concepts, for instance:
power structures OR gender OR race
"California Missions" OR "Mission San Gabriel"
"Alta California" OR Mexico
Use quotation marks to search for a phrase
Use a symbol (* is the most common) to look for variant endings of a word