As someone who wishes to become an artist and practice anatomy, I think I could help you out. First, you need a basic understanding of a normal human's anatomy, you can draw it in a stylized way if desired, but it is important to have the basics down. Some books can be really helpful when it comes to this, one that personally really helped is "Freehand figure Drawing for Illustrators" by David H. Ross.
Once you've mastered that, you can add any special features or designs you wish to include. For a spider, you might want to look at others take on that. Of course I'm not saying you should steal a person's design, but it could give you some much needed inspiration.
I hope this helped!