In the method section, you have to describe your search strategy, but also your criteria for selection and assessment of relevance. In other words, how you after the search choose the sources that you will analyze in more depth.
The assessment of relevance may be based on for example your hypothesis or the hypothesis in the sources, reading of title/abstract, presence of keywords in the sources, type of study (e.g. qualitative or quantitative) or the method used in the sources. Some people use a point system to classify the quality of the sources based on certain criteria. In this way you can get a relatively independent (or at least transparent) measurement of the quality of the sources.
The selection can be presented in a figure:
