Every mod runs on GDPatch: put gdpatch_loader.dll next to CICADAMATA.exe and rename it winmm.dll.
The mods hub, from the releases page. Unzip it into the game folder: settings show under Options > MODS.
Steam: right-click CICADAMATA > Manage > Browse local files.
Zips already hold GDPatch\mods\<mod>. CASCADE, SKIN KIT and every mod drop in the same way.
Finish the campaign once: EXTRA shows LEVEL EDITOR and CASCADE. Mods live under EXTRA > CASCADE > MODS.