You need to have a Steam account and connect it to your store account. The skins you buy will be automatically added to your Team Fortress 2 inventory. Once they are there, you can use them in the game to customize your characters.

By using our skins you can customize your characters’ equipment, giving them a distinct appearance and adding variety to the gameplay. You can alter the textures of weapons, skins, accessories, taunts, and tools. Your creativity is the only limit to how unique your characters will look, what weapons they will wield in battle, and how they will stand out among teammates and opponents.