there are already many topics on this
- Soil moisture mapping using Sentinel-1-Data and SNAP?
- Soil Moisture Mapping Using S1_SLC product and s1tx
- Soil moisture identification -Sentinel
- S1 GRD product - retrieve soil moisture
- Backscattering for Soil Moisture
You will soon see that there are many approaches of doing it. Many are based on field measurements and images from both rainy and dry periods.