Find a Nether Fortress in the Nether
Collect wither skeleton skulls from Wither Skeletons
Collect soul sand or soul soil
Build a Wither using 4 soul sand or soul soil and 3 wither skeleton skulls
Defeat the Wither
Pick up the Nether Star it drops
Find a Nether Fortress in the Nether
Collect wither skeleton skulls from Wither Skeletons
Collect soul sand or soul soil
Build a Wither using 4 soul sand or soul soil and 3 wither skeleton skulls
Defeat the Wither
Pick up the Nether Star it drops