Is it a step by step procedure which can be adopted for final liberation or just grace of God?

Both. The grace delivers the liberation only when one reaches, by one’s own efforts, the brim of purity. As I often say, this is similar to the titration experiments in chemistry classes. Each drop counts, and is important for the transformation. But it is the last drop which is the grace that brings in the final change, and you never know when this would happen.