Staying true to early desktop icon dimensions, create your art on a 32×32 pixel canvas. 72 pixels per inch will do nicely as well.
Limit colors to the 256 Mac OS palette. Download the DPS color picker image below or choose the “MAC OS” swatches within Photoshop.
Enlarge to 960px square, with resampling set to Nearest Neighbor. Export your image with the lossless PNG format and you’re done!