You create cosmetics from within programs like Blender or 3DS Max. The smd files are already on your computer. Navigate to steam/steamapps/common/sourcesdk_content/tf/modelsrc/player/demo/parts/dmx. There you will find the demo_model.smd file you'd want to use. You'll need the Source SDK for this, get this by going to Steam, then hover over Library until a menu appears. Look for the "Tools" menu, then click it. Scroll down to the Source SDK name, then double click it. It'll now install.