Here are 8 steps how to go into it:

Choose some drawing/pixel art software and doodle to get used to the four basic tools.

Play around with advanced tools as well to know what’s available to be more efficient.

Copy an existing pixel art sprite by looking at it as a reference.

Learn about jaggies and clean lines in pixel art. Draw a black & white sketch of a landscape/cityscape in side view. Here you might spend some time learning how to sketch (with pixels or pencils) in side view to get a hang of it.

Learn about limited color palettes and recolor an old screenshot such as from a Game Boy game.

Learn about color banding and color ramps and practice shading different materials.

Learn about clusters and anti-aliasing and copy a photograph reference into pixel art. You’re just combining lineart drawing from step 4 with colors/shading from steps 5–6. You can simply trace the lineart from a photograph or first learn how to copy a reference with a grid or sighting.

Learn about dithering and different outline styles. Try to make a copy of a reference with more limitations or put a personal style to it. Here’s a couple of artworks drawn from reference with old computer limitations or palettes (NES, ZX Spectrum, Atari 2600).