Here's a quick forearm muscle study using my pal and fellow illustration instructor, Rick Kryczka. We see mostly the extensor/supinator group here, which is discussed more thoroughly in The Dorsal Forearm Part 1: Compartment Search, The Dorsal Forearm Part 2: Which Side Are You On, Anyway?, and The Dorsal Forearm Part 3: The Final Chapter. There are also smaller updates on this area in The Dorsal Forearm: One Last Encore and Landmark Sightings Part 1: Bruce Lee.
You might start orienting yourself on this image by first identifying the olecranon process of the ulna and the lateral epicondyle of the humerus. From there you can identify anconeus, the small triangular elbow muscle that originates between those two landmarks and then points into the crease between the extensor and flexor forearm muscle groups groups. (And if you keep following it distally, will take you to the crest of the ulna.) From there you can identify the twin muscles (extensor carpi ulnaris and extensor digitorum) and then extensor digiti minimi between them, and so on. The links mentioned above explain all this in much greater detail.