My process is simple. I sketched everything on a raster-app and what I am using is Medibang Paint Pro, it's totally free and light weight than other counterpart like Photoshop, Corelpainter etc. I conceptualized and I sketched my design based on a body flow of an ancient greek statue but do a 'C' formation on its hand and feet. Finalized it on Illustrator to have a vector output using vector lines style for sketch look.