Answered a similar question earlier so I’ll just copy my response.
One thing that helped me learn, aside from a couple books and tutorials, was I went to Spriters Resource and downloaded a bunch of those sprites to study. I would zoom in on them and deconstruct them to learn how to create some of those effects. Then I would attempt to color edit them manually, or add details to them to see what worked or what didn’t.
Then I would find images or things in my house and try to recreate them. When I felt I was getting OK I joined a community and would post my work for experienced spriters to critique and they would help find flaws and give advice.
Spriting small art and large also take different techniques, if you are doing something small you have to use certain tricks to squeeze the detail out of your image.If you go really large there is plenty of room for detail but more pixels to blend to create effects so that you don’t get harsh lines.
I will share one technique I learned way too late was hue shifting. Learning that really made my art pop. Learn how the colors and hues work. It will make a world of difference in your art.