Most sockets on "Diablo II: Lord of Destruction" items occur naturally in place of other magical effects, but there are two ways to add one to an item. This allows you to give a powerful rare, set, crafted or unique item an extra boost. The first quest in the fifth act offers a free socket, and the Horadric Cube has a recipe for adding a socket to a rare item. The Cube also has recipes for turning normal items into socketed items, priming them for runewords.
Siege on Harrogath
Larzuk in Act 5 offers the quest "Siege on Harrogath," which requires you to slay Shenk the Overseer. This boss appears at the end of the Bloody Foothills, the first area. After beating Shenk, return to Larzuk to add a socket to an item. You can only complete this quest once per difficulty, so save the reward until you have a worthy item. Larzuk can add a single socket to any rare, set, crafted or unique gear. To earn extra sockets, hand off an item to a newly created character and rush through the game to the fifth act. Even when skipping optional quests, however, this adventure takes a few hours.
Horadric Cube Socketing
The Horadric Cube from Act 2 has a vastly overpriced recipe to socket rare items: Insert a perfect skull and the unique ring Stone of Jordan along with a rare to add a single socket. The Cube can also add a random number of sockets to normal items. The recipe varies by type: Weapons require a perfect amethyst, a Ral rune and an Amn rune. Armor takes a perfect topaz with Tal and Thul runes. Helms need a perfect sapphire with Ral and Thul, and shields need a perfect ruby with Tal and Amn.