Minecraft: How to make a Nether Portal
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14 Obsidian (Minimum Size, Note: Since 1.10ish Nether Portals can have anysize except smaller than 4x5)
10 Obsidian can be used for the Smallest.
A = Obsidian
B = Air
B A A B A B B A A B B A A B B A <-- Here is where you use Flint and Steel in the B's B A A B
The 4 B's on each corner can be replaced with Obsidian but it will cost 4 extra Obsidian though. (Not needed though)