Nail the muscles
From here, I begin to define muscle groups, being sure to understand where certain muscles are connecting. This is vital and is informed by producing lots of animal drawing. It’s important to know where muscles lie on your creatures for animation and modelling. Being able to show and tell where and how your creature moves is crucial when creating a grounded creature design.
Remember that vertebrate anatomy is similar between species. You can think of your own body as a reference point. Keep the shapes big and light at this point. Think about where the muscles would attach to the bone.