I don't know if it's possible to describe in any detail which aspects you should focus on without knowing something about what you're writing in the chapter, since as Sverre said, it's ultimately just the answer to one question.
An introduction is ultimately just a concise summary of the contents of a piece of writing, specifically a chapter; in this case, the points to focus on would be the basic components of a description of research methodology:
- Reminder: We are researching...
- The usual methodologies used in this field are...
- X, Y and Z were considered for use.
- X, while [pros], [cons], whereas Y [pros], but [cons], and Z [pros], but [cons].
- Due to the importance of... ...in this study, we chose to use Y.
Then you can explain your full reasoning in the remainder of the chapter.