You’ve learned the basics and had a look at the first steps. Now it’s time to learn about making your own. For me, a finished piece of pixel art often starts with a sketch. It’s entirely possible to just start without first making a sketch to work off of, but in my opinion you can never go wrong with laying down your foundation first. But how do you go about preparing a sketch, whether it’s scanned or drawn digitally, for making into pixel art?
Sketching

Preparing a sketch for pixel art actually already starts while you’re sketching. It’s very important to keep your final product in mind: picture it in your head, lay the foundations for it, and work towards that final product. Being able to envision it in detail is a skill which will help you immensely.
Sketches which will be turned into pixel art are different from sketches which will be turned into digital paintings. Pixel art is small, which means that small details will end up disappearing or being completely unreadable on a small size. So, stick to bigger details!
Of course the bigger the piece of pixel art you’re planning, the more details you can put in. If your final product will be on a canvas size of 500 by 500 pixels (not recommended, by the way – far too much space to fill in), you can put in a high amount of detail. But at more common sizes like 32 by 32 pixels or even smaller, you’ll be a lot more restricted – there’s limited space and you’ll want to capture the essence, rather than the specifics.
Cleaning up

Once you’re happy with it, the next step in preparing your sketch is to clean it up. If your sketch is on paper, I recommend scanning it now and doing the cleanup in Photoshop or your preferred program (MS Paint and Aseprite aren’t really suitable for this, although it is still possible if you really want to use them).
Any lines that are somewhat weak and thin, make them stronger and thicker. Get rid of construction lines, non-essential lines and lines left over from sketching. Small lines will mostly disappear during resizing, but can still cause some confusion at a small scale.
The final part of cleaning up is to crop the edges of your canvas as close to the actual lines as possible.
Finishing up
You’re close to done now, but you still have a large image. The final step of preparing a sketch is to resize it.
In Photoshop, go to Image -> Image Size and enter the size you want the sprite to be. Other programs should have similar methods. While determining the size, keep in mind that you won’t be able to resize your piece easily once you start on it, so think about it very carefully! Try a few different sizes if you need and pick the one that works best.

Now that the sketch is smaller, some lines may be somewhat faint. This happens especially when they’re on a transparent background. You can draw over them with a very small brush to make them thicker if needed. I usually have transparent backgrounds, so what I do is simply duplicate the layer with the lines until they’re as visible as I want them to be. Adding a background layer behind them can also work; if you do this I recommend going for a neutral grey, since this will have the least amount of influence on your vision when you start on colours.
And that’s all there is to it! You are now done with preparing the sketch, and can move on to drawing the sprite outlines.