Downscaling + Indexing Game Assets 2023-05-10

(Apologies if I did a poor job explaining, this is the first tutorial resource I've made!)
While we're in the age that isn't bound to the same restrictions that the older consoles had, I'm a sucker for having things be authentic towards the original source material. Some may remember that older games actually used downscaled art or logos rather than proper pixel art, which can be common for games that were ported to many systems (like Puyo POP), or just simply a downscaled logo like we see in games such as Pokémon most of the time (for things like title screens!).

The question we'd essentially be asking is...

Disclaimer: For this tutorial, we'd be using Aseprite. Other programs may be able to do this as well, but this is really the most comfortable program for me. It has some convenient features such as being able to quickly make a palette out of what's given to it, which is what will be used in this tutorial.

While I could use some art for this, I'll be using a logo for the game I'm developing instead. Art is a different story to this that I'll explain after I cover the basics.
Step 1: Open up the logo in Aseprite (or your preferred art program). As we can see at first glance, Aseprite tries to generate a palette from the image but since there are too many colors it sort of just... Overflows. This is typically normal and not really much of a big deal; we'll be dealing with that regardless.
Step 2: Click the drop-down menu above the palette and select "New Palette from Sprite".
From here, we can now see that we have the option of selecting how big the palette would be! How convenient. That number's up to your preference, but for the sake of my logo I'll be setting it to 32.
Now then, with that done, you can see that Aseprite has generated a palette of your choice of colors. It's not gonna be in any fancy order; but, it wouldn't really matter either way. What matters is that a palette was generated. (Technically there are 31 colors here, but since PNGs are transparent the 32nd color is made into transparency.)
Step 3: It's time to properly downscale it. Click on Sprite then navigate to Sprite Size...
From here, a menu will come up asking what size you'd like to change the sprite to, as well as what interpolation method the scaling will use.
The interpolation method matters a lot. This isn't a simple question that can be answered; it really depends on what you're downscaling. More complex art may benefit from bilinear, and more simple logos may benefit from nearest neighbor. Experiment and see what you like more. It's okay if you don't find something you like right away. For the sake of this tutorial (and my own taste), I'll be using nearest neighbor for the Bravery logo.
(Also, just a side note, RotSprite is meant more for rotating sprites in Aseprite. It'd most likely be identical to nearest neighbor.)
Step 4: Indexing your logo is quite simple. Navigate back to the Sprite tab again, and hover over Color Mode. You'll see 3 options presented to you; out of the three, select Indexed.
That's pretty much the gist of making game logos look like how it did on the DS. If anything, I'd recommend changing the setting back to RGB so that you don't have any sort of technical issues with it later on.
You don't have to make a palette just from the image alone as well. Have a specific palette made for your game? Put it into Aseprite and index it like that!

Game art on the other hand is a slightly different story.
While logos are simple to convert, game art is a bit weird. As much fun as it is to have it be crusty, it really is quite jarring to see something like this:
For context, on the left is the official sprite for Amitie in Puyo POP Fever (DS). On the right is a downscaled version of the official art made using this method. There's an obvious difference in quality. While it's unknown as to how SEGA pulled it off, it's safe to assume that there's a lot of cleanup involved when doing something like this.
When downscaling game art, remember to clean it up and make it more accurate to the original art at hand. It's best not to be lazy with it or else you'd get something truly crusty to look at.

Hopefully this helped a lot! There's not much else I can add to this topic, or at least not much that I know of.