Nether Star
Nether Stars are a drop from the Wither boss, which is summoned and defeated in the same way as vanilla. Modded offers a number of additional tools for easier Wither/Nether Star farming, including bypassing the Wither fight completely after at least one Nether Star is obtained.
Nether Star Duplication
If you don’t want to farm multiple Withers for Nether Stars, they can be duped using Salis Mundis, which requires a basic Thaumcraft setup (first wand and cauldron). To make Salis Mundis, create Balanced Shards in the Thaumcraft Crucible or Thaumatorium. This requires at least one type of elemental Shard from the Twilight Forest, combined with all the other five basic aspects not in the shard (Terra, Aer, Ignis, Aqua, Ordo and Perditio). The Balanced Shards are then processed in a Macerator to produce Salis Mundis, which is then combined with Null Catalyst from Witchery in the Assembler. An initial macerated Nether Star is necessary to start the process, as well as a good supply of slimeballs and blaze powder. A cheaper recursive recipe is available after making the first catalyst. Make sure to save at least one Null Catalyst to continue duplicating it.
Skullfire Sword
This sword has the special property of being able to drop Wither Skulls from normal skeletons at the rate of three per kill. Easily the fastest Wither Skull farming option. Requires an Avaritia Dire Crafting Table and some Thaumcraft progress.
Cleaver
A Tinker's Construct Cleaver has the natural property of Beheading, which increases skull drop odds to 20% (not just Wither Skulls). Additional levels of Beheading can be added to Cleavers (or other TiCon weapons) at the cost of a modifier, ender pearl and obsidian for an additional +10% chance. Combine with a skeleton spawner in the Nether for an easy Wither Skeleton mob farm.
Witherproof Box
Reinforced Obsidian from EnderIO can be used to construct a cage or box to spawn the Wither in from which it cannot escape. While an actual box works, as few as fifteen blocks will suffice to build a structure that can hold it. The Wither will still do AoE damage on spawning, so build somewhere away from base or surround with blastproof blocks. A single layer of blocks above the Wither's head may not be enough to keep it contained, so a second layer is strongly recommended.