Find and Collect Obsidian
Why You Need Obsidian
Obsidian is essential for building a Nether portal. It won't spawn naturally on the islands, so you must search carefully or create it yourself.
Creating Obsidian
The easiest method is to make obsidian using lava and water. Dig a deep hole or channel, pour lava into it, then pour water over the lava from above. Obsidian forms where they meet. Harvest it with a diamond pickaxe or better—stone and iron pickaxes won't work.
Collecting Enough Blocks
You need at least 10 obsidian blocks to build a complete Nether portal frame (14 blocks total, but you can use other blocks for corners). Mine multiple obsidian blocks and store them safely.
Portal Construction
Once you have enough obsidian, build a rectangular frame on one of the islands or your dirt block. The frame should be 4 blocks wide and 5 blocks tall. Light it with flint and steel or fire to activate the portal and enter the Nether.