Now that you’ve watched me demo the skeleton and skulls of apes. It's time to draw them yourself using the 3d models and photos provided.
Focus on obtaining the fundamental knowledge of these sources. The more you understand the source material, the more believable your imaginative work will be.
Do multiple sketches from different angles and try not to do the same angles I covered in the demo.
Once you're done sketching from different angles, try some memory sketching. Take the concepts you just practiced and try to sketch them again from memory only. This will help you find the areas you are exceeding and lacking in.