The Graphic Design
Arguably the most important part of the thumbnail process is finding a strong overall design. This is a balance or harmony of shapes and values. Before any details, an appealing organization of shapes and values is what I'm going for. The whole reason of doing the sketches at thumbnail size is to have better visual control over this. It keeps us from getting into detail too soon.
These thumbnails go a bit further with some detail, but you can see that they each have a strong graphic design at a small scale. I know that each of these will work on a design level. This phase will eventually become somewhat intuitive and you'll be able to have the Graphic Design mentality as you start your thumbnails.
Here is where I start thinking about the elements of the image: Perspective, camera angle, size relationships, foreground-middle ground-background, and placement of the elements. I try and stick to my initial graphic design as much as possible.
Often times I group the Graphic Design phase with this one. I know what the image needs based on an idea or assignment and I organize the elements into a strong graphic design as I explore different possibilities through thumbnailing.
The Value Break-up
The thumbnails above have a rough value break-up but I want to be a little more clear with how I'm going to organize values. I try and focus on just 3 values: Light, middle, and dark. There is a great chapter in "The Famous Artist Course" that goes into detail about this. I highly recommend reading through that, if you can find it. You want to find ways to bring attention to your focus by controlling your values. Contrast is one of the best ways to get the viewer to look where you want them to. I will often place a light value behind a dark figure or a dark value behind a light figure if I want that figure to stand out.
There are different combinations and value ratios you can play with:
Light on top of Middle on top of Dark
Dark on top of Light on top of Middle
Middle on top of Dark on top of Light (thumbnail B below)
Dark on top of Middle on top of Light (thumbnail A, C, and E below)
Light on top of Dark on top of Middle (thumbnail D below)
ColorAdding color can be a great way of getting a better feel for the mood and give your AD a better idea of your intentions for the final piece. There are hundreds of great color combinations. Most combinations have already been invented so I'm mainly looking for color harmonies that will best support the emotion I'm trying to evoke in the image.
The color sketches above go a little beyond the thumbnail tag. However, both of these started out like the thumbnails above. Thumbnails are just for me, a chance to explore as many variations as possible in a short amount of time. This is what I would give my AD for approval to move to a final. There is enough detail and color information there to let the AD make any comments or approve to move onto final.