Copy and Paste or Drag and Drop the Data folder into your Skyrim Directory and click 'yes' to overwrite if/when required.

Optional:

The "Radiant Clothes.esp" file in the "Optional" directory replaces the "Radiant Raiment" dress and the "Party Clothes" (given to the player by Delphine during the main quest line before going to the High Elf Embassy) with the included "Radiant" variant of the "FineClothes01" dress. Copy/Paste or Drag and Drop the Radiant Clothes.esp file into your Skyrim\Data directory and enable the mod if/when required.