To build a Nether portal in Minecraft, you need at least 10 obsidian blocks. This minimum frame is a vertical rectangle that is 4 blocks wide and 5 blocks tall, but the corners are not required, so you can omit the 4 corner blocks and use other blocks instead if you want to save obsidian. Using all 14 obsidian blocks (including corners) makes building the frame easier and looks more complete, but only 10 obsidian blocks are necessary to make a functional portal. Once the frame is built, you activate it by lighting it on fire with a flint and steel, which fills the frame with a purple portal texture