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Step 1: Cobblestone
Place five cobblestone blocks in a cross like this.
Step 2: Gold
Place four gold blocks in the. Corners of the cobblestone cross.
Step 3: Nether Reactor Core
Place a nether reactor core on top of the cobblestone block in the middle of the cross.
Step 4: Cross
Put five more cobblestone blocks in the shape of a cross on top of the nether reactor core.
Step 5: Corners
Put one cobblestone block on top of each gold blocks. So you need four cobblestone blocks because there are four gold blocks.