The sprites and code here do not serve as a good example; a new mod repository will be made for V6 that showcases more modding features.
Simply download this repository as a zip, then import it through the Mods dialog in Mindustry. Or, unzip this repo inside Mindustry's mods/ folder.
Feel free to submit more example content to this repository. For example sprites, I recommend re-coloring existing blocks and using the existing Mindustry palette.
Simon Woodburry-Forget has a list here. If you want your mod on there, you can add it here.