Establish Approach or Methodology
You now need to decide the overall approach or methodology that will be used in your project.
For example, you may want to assess historical trends and develop projections of monthly rainfall of a particular region. For assessing historical trends you may use observed rainfall data. You may choose to apply the exploratory data analysis (EDA) to produce graphs as well as to explore, understand and present data, and subsequently apply a given statistical trend analysis such as the Mann-Kendall test.
If, for instance, you decide to undertake the analysis using observed station data, you may then need to decide how many rainfall sites will be included, and if you will do analysis on each of the stations or on an area-average based on rainfall value of all the stations.
For developing rainfall projections you may decide to compare climate model outputs for future time periods against those for the baseline period. You may also determine to produce time series plot and/or apply trend analyses to climate model time series data
You may also need to think about how many and which climate models you can analyze with your available resources (step 6), and if you will evaluate climate model output by yourself or by considering previous work (step 5).
Be mindful that establishing a methodology is not always straightforward. Sometimes you may need to re-adjust your methodology to consider feedback or new learnings from subsequent steps.
 For example, Kundzewicz ZW, Robson AJ. 2004. Change detection in hydrological records – a review of the methodology. Hydrological Sciences Journal, 49, 7–19, doi:10.1623/hysj.18.104.22.168993