It's going to sound cliched, but I've been at this for like, 7 years. I started out editing sprites from Jump Ultimate Stars, and eventually starting working on my own stuff. I remember early on, and still kind of to this day, looking at stuff I liked and trying to go for it. For example, a lot of my work going back was based on Hyper Light Drifter's art style; even now, stuff like my RWBY animations utilize that simplistic but striking pixel density. After that, I slowly started picking up stuff like key frames, squash and stretch, and smearing. I'm still working on larger scale stuff and things like anatomy, and I use a program called DesignDoll to help me figure that out.
I haven't actually looked too in depth to it just yet, but someone pointed me towards this:
www.patreon.com/saint11 It's a master explaining different aspects of pixel art that I should use to review a bit.