There is no secret to do this, it is only a question of being persevering, for example in adapting to
ED-209, I made 35 attempts before deciding which would be the best method to edit it.
When it comes to smaller graphics, what I do is work without restrictions, and then adapt them to the style I need.

Sometimes I draw sprites much larger than necessary to better understand the structure I want to represent.

But always the biggest challenge is the reduction of colors, personally I like to do it manually, to get the best results.

But luckily there are also automatic methods that are very effective.
Firework MX and
Color Cuantizer are the best tools for this purpose.
Take for example again
ED-209, to speed up that work, I did the following.
I sought to create a range of colors higher than necessary, in this case with 5 colors seemed sufficient to me.

After this save three versions:
One as a reference with the
5 colors and then another 2 with a range of
4 colors, where in one remove the darkest and another instead the lighter color.

Then I loaded these images to Photoshop and matched their colors using a unique palette for both images.

In this way later I made a single image from the two files, with the help of the reference.
(This work is done with much more care)Other useful programs:
ImageResizer-r129: It allows you to resize images with all existing methods of interpretation.
Rotsprite: It serves to rotate objects in a unique way.
Tiled: Its uses are almost endless.
PhotoScape + DoubleKiller: It is the best way to detect repeated graphics.
● With the PhotoScape Cutter tool, you can split a tab into 8x8 graphics, then with DoubleKiller delete the repeated sprites and then with the Mural tool (also from PhotoScape) reassemble the image.