Instructions
Place a custom marker on your map to mark the location of Lydia's body.
Raise your Conjuration skill to level 90. To quickly raise your level, cast cheap Conjuration spells, such as Bound Sword or Raise Zombie. You can also gain levels by reading skill books, including The Warrior's Charge, Liminal Bridges and The Doors of Oblivion. Another method is to repeatedly cast Soul Trap on your horse or a corpse.
Go to the College of Winterhold and talk to Phinis Gestor. Ask him about furthering your Conjuration magic studies to get the "Conjuration Ritual" quest.
Enter the Hall of Attainment and go to the roof. Cast the "Summon Unbound Dremora" spell.
Tell the Dremora that you control it now. When it resists, kill it and then re-summon it. Once again, demand that the Dremora serve you. Kill it a second time. Re-summon it once more and the Dremora will agree to submit to you.
Summon the Dremora again and take the Sigil Stone from it. Take the item to Phinis to receive the Summon Flame Thrall spell tome and unlock the Dead Thrall spell for purchase.
Talk to Phinis again and buy the Dead Thrall spell from him. Read the book he gives you to learn the spell.
Return to Lydia's body. If you want her to carry any items, such as a shield or weapon, place them on her corpse before reviving her. Lyida will only wear the armor in which she died even if you give her new armor.
Cast Dead Thrall on Lydia. Dead Thrall allows Lydia to fight with you until she's killed. If she dies, cast the spell on her again to revive her.