Choose a tool for Image Manipulation - Paint 3D is standard on windows, GIMP is more advanced
setup GIMP - you'll be needing this for colour reduction
setup PNGgauntlet - you'll be needing this for filesize reduction
setup Emage - a PNG optimized file, can be further compressed using Emage
setup a directory for downloads & intial image creation, a directory for GIMP exports, a directory for PNG optimized files. This way your work follows a logical flow, and you don't accidentally mix up or upload files from previous steps