Seeds and portals
First, you have to make a world. So press the "new" button and press advanced in the top corner. Then press "seed" and type your seed. [999,kick,etc]. After that,pick a type of world: old, infinite, or flat. Then name your world name it anything. Then, just for now press creative. Now go to the three dots and get obsidian and water. Then place a rectangle 4 by 6 then put water on the inside top 2 you are in the nethier. If you want to build a portal to the Wither that's for you to find out!