how to convert image to template

Resizing image to a certain size:
image > scale image
interpolation none if you don't want a blurry image

Creating the palette:
open this image in gimp (https://files.catbox.moe/suegev.png)
windows > dockable dialogs > palettes
right click the palettes window > import
import from image
Now you have the palette in gimp. Rename it to pixelcanvas for instance.

To force the palette onto an image:
open the image
image > mode > indexed
use custom palette
select pixelcanvas palette
dithering method > floyd-steinberg