Thumbnailing is a common technique in concept art to create a lot of ideas quickly. You draw a series of thumbnail sized images that focus on the silhouette and major shapes. Here is an example of a few thumbnails I made for cat buildings.
Thumbnails are very useful for trying out a lot of very different ideas, and works well with subjects that are more abstract. They often result in a final concept that has a very interesting overall form, but I also feel that thumbnails have some drawbacks, which is why I often skip the step.
There is a tendency with thumbnailed concepts to emphasize the large shapes over everything else. Often people will create 20 or more thumbnail images and pick the ones that stand out, which inevitably leads to picking the most outlandish of the designs. Things like the pose of the character in the thumbnail image may make one design seem a lot cooler than another even though the pose is not important at this stage in the process.
There are a lot of different ways to design a concept. You could, for example, imagine you are an engineer and try to design something that you think would work. You can take an existing real life design and work from that as a basis. You can use bits and pieces of images that you arrange until you find an appealing shape. There is no right or wrong method, but whatever method you choose will result in a different emphasis in the final design. It is important to keep that in mind when you choose how you will design something.