The introduction presents the question being asked and places it in the context of what is already known about the topic. Background information which suggests why the topic is of interest and related findings by other investigators are mentioned. This section is typically two pages. The methods should be is sufficient detail so that others can reproduce your research. This section of the paper should describe the materials and procedures used in sufficient detail that others could repeat the research if they wanted to see if they could get similar results.
Reproducibility is a key aspect of scientific research.
Specific, published techniques can be cited without being described in detail. However, if the published techniques are an essential part of the research, then you are obligated to describe them at least briefly. It is common to narrate the methods in the past tense, telling what was done, not telling what the reader should do. Diagrams and tables are often helpful. One to three pages are usually adequate.