Download the Mod
Go to Planet Elder Scroll's "Oblivion" mod section and locate Jpm's Miniature Necromancy Mod (see References for link). The download link is very small and near the bottom of the page, making it difficult to find. However, if you click the name of the mod, your screen will immediately jump down to the download link.
Install the Mod
Once Jpm's Miniature Necromancy Mod is downloaded, decompress the file. WinZip, WinRa, and FreeZip all can decompress the file if you do not already have a file compression program on your computer. Once the file is decompressed, navigate to the "Oblivion" folder on your hard drive and open the folder called "Data." Copy and paste the files from the decompressed mod folder to the "Data" folder, then close everything.
Double-click the "Oblivion" icon to start the game, but when the launcher appears, press the "Data Files" button instead of "Play." A new window will appear. In the left-hand side of the window will appear all the files the game will use. Make sure Oblivion.esm and Jpm's Miniature Necromancy.esp have a check mark in the box next to them. Press the "OK" button, then press "Play" when the launcher reappears.
Becoming a Necromancer
Go to the Arch-Mage's lobby in Arcane University. A new book will be in there that wasn't there on your previous trips. If you read that book, all five Necromantic spells available for the class will be added to your character. These include the ability to summon a skeleton minion, a zombie minion, a ghost minion, the ability to revive a fallen enemy as your minion, and the "Harvest the Dead" spell which strips a dead body of all its treasure and human body parts to fashion new minions.
Speak to the High Elf
Just outside the doors of Arcane University, you will find a high elf wearing the distinctive Necromancer robes, which are black with a red emblem on the breast. If you are a Necromancer, he will ask you to join him as part of the University's secret Necromancer sect, and will sell you, at exorbitant prices, the materials to practice your craft.