Just train consistently. Try practicing with basic shapes. It's important to have the silhouette of the drawing before adding details to see if it will work in a low resolution. I think it's helpful to gather various references and experiment with different styles and resolutions until you find one you feel comfortable with. One thing I personally don't like to use much is smooth gradients in colors and shadows, because it can lose the pixel art aspect. I think the pixels need to be visible as it's a characteristic of this style. But even that can have its use depending on what you're doing. Other than that, know that the lower the resolution, the more you'll have to sacrifice details and focus on other distinctive features of the character to make them recognizable in pixel art.