You'll notice that in my sketch, I leave a lot to the imagination; a few scraggy lines denote an ice mountain range, a few scribbles underneath the creature's head mark the position where I will illustrate a nest. The important thing to keep in mind at this stage is balance - does the image seem balanced? If not, shift some elements around until it does. Often it takes a bit of trial, error and experience which you only get from getting down and dirty.
This process of shifting elements is facilitated by rough abstract shapes and I only put in details where I need to work something out.
You can observe in my sketch that the face/beak area is where I've spent some time adding extra details because this will be the area of focus. That being said, it is still quite rough and will undergo quite a lot of changes as the image progresses.
Create a new layer set to Multiply and drop a cool mid-tone. I have chosen a mid-tone cool hue wash as it allows me to better gauge my highlights and shadow and because I also want a predominantly cool image (Fig.02). I have also started to paint in some darker tones to denote form and establish an idea of where the key light should probably be coming from. During this phase of the illustration, I am using the Soft Round brush that comes standard with Photoshop. This brush allows me to block out large swaths of tone and stops me from starting on the details too early in the process.
Using the mid-tone as a good starting point, paint in the darkest tones and the brightest highlights coming from the sun (which is outside of the image). You can also see that I'm starting to tighten the rendering around the facial area relatively quickly to establish a baseline quality against which the rest of the image will be judged. In particular I am beginning to alter the silhouette of the head and add more weight to the bottom of the griffin's lower beak to counterbalance the heaviness of the top part of the head. In the background, start to drop in some darker tones to show shadow areas on what will eventually become big ice shards/mountains (Fig.03).
Once you're happy, press Ctrl + Shift + Alt + E to create a summary layer of all the layers underneath. Next we're going to begin a process of lassoing and shifting various picture elements around. I have darkened the underlying image on the left to show you the two main areas I am repositioning - the eye and the ear (Fig.04).
We don't want to concentrate all of our efforts in one area, so I'm going to begin building up the surrounding elements to allow the image to advance as a whole. This allows us to judge whether it is progressing in the desired direction. In particular, start putting in the highlights on the ice in the background and adding more line work design to the wing area. I am using a combination of the Soft Round brush and a custom Chalk brush I have created that simulates the feel of drawing with a pencil (Fig.05).