Quick Sketch Ractice

I did 2 quick sketch practice during the weekend, one is a shirt (take 3 hours) that referencing from my past black burn shirt work for battlefield 1. the other one is a monster practice (take 1.5 hours) without any concept or reference, just let my hands go freely, not very satisfied with what I get in the end, but I would willing to share some personally sculpting methods, Thanks.