The goal of your methodology is to increase the credibility of your research by making your process transparent and reproducible. Your methodology should begin by describing your research question and the type of data you used in answering it. You want to indicate why this type of data is appropriate, relevant, and important to the question being asked. You will then explain your process of data collection. This is the place to state the tools and materials that were used to collect data, the process and criteria you used to sample subjects, and the size of your sample. If your research was quantitative, you will specify how your data was measured. If your research was qualitative, you will describe how data was recorded and how you observed and/or participated in the study. You want to provide enough detail for readers to be able to replicate your procedure. If you used existing data sets instead of collecting new data, you can elaborate on where you found the data sets, how the set was originally produced, and what criteria you used to select the data. Remember to explain why you made these decisions.

It depends on the field of study.

In short, it should include (I use the following sequence/headings)....

Chapter introduction

A short paragraph on defining research methodology

Recap of study questions/objectives

Research paradigm e.g – positivism, constructivism etc

Research design or approach – e.g. qualitative or quantitative, mixed methods etc

Location of Study

Population and sampling techniques

Data Collection - Including how the instrument/s were designed and administered

Validity and reliability

Data Analysis - what tests did you perform or foresee performing (quantitative and/or qualitative etc)

Ethical Considerations

Limitations

Conclusion

Remember you have to provide justification with references for the your chosen research approaches (per points/headings above)

I agree that the format for this differs widely across fields. So, you should select 3-4 articles that use a similar methodology to your own study, and then create a detailed outline of the relevant section from each of them.