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To create a Nether Portal, you'll need at least ten obsidian blocks. Make a hollow standing rectangle, 5 blocks high and 4 blocks long (corners are not necessary; you can use any block as a stand-in). Then strike the inside of the rectangle with flint and steel. The empty inner space should turn purple and start to make interesting sounds. Stand in the portal and wait 4 seconds to be transported into the Nether. Make sure you have your flint and steel for relighting the portal in case it breaks from a ghast and plenty of food to keep yourself well-fed. A Nether Fortress is a structure in the Nether that contains Nether Wart and a Blaze spawner.  They can be somewhat hard to find, so it could take a while. Blazes drop Blaze rods, which are needed for making potions and getting to the endgame. You will need a lot of Blaze rods. Nether Wart is a key ingredient in potions, and it only grows on Soul Sand, so collect some Soul Sand (it's brown with a face-like pattern) and try farming it in your base. Nether Wart can be found naturally occurring in nether fortresses in two small patches alongside stairs going upwards. This will let you brew potions, which will help during the final boss fight.
Build and light a Nether Portal. Enter the Nether. Find a Nether Fortress. Kill Blazes and gather Nether Wart. Craft a brewing stand.