As CD Projekt Red promised before the game launched, The Witcher 3 received mod tools last week. This was done in partnership with Nexus Mods, allowing users to install and configure user content using the Nexus Mod Manager. Note that this only applies to mods that were created with the official mod kit, not any arbitrary mod that has been created since release.
CD Projekt Red has also provided four simple samples (serves you right for trying to read my news posts aloud) as a tutorial. “Witcher The Slav” retextures “Geralt's starting outfit”. “Fabulous Roach” modifies the meshes and textures that make up Geralt's horse. “Petard sWitcher”, and the more mundanely named Custom Equipment Sets Mod, each introduces game scripting concepts.
Go forth and mod.