Break the second to last and third to last blocks from the edge, above the exit pipe in World 1-2, while you are Super or Fire Power Mario. Then walk as far right as you can, then as far to the left. Repeat this procedure until you can see the edge of the warp pipe on the right edge of the screen. Stand with your trailing foot hanging over the edge of the exit pipe, and with your other foot on the pipe. Then press Down to crouch. Hold Down, press A, and let Mario jump straight up while still being crouched. Just before Mario reaches the peak of his jump, hold Right, which should barely brushing the back of Mario's head on the remaining block above. Mario will be able to walk through the wall and enter the warp area. There will be Piranha Plants in the pipes when you first enter the area. Before walking to the far right of the warp zone (which will allow the normal warp zones to appear) enter the far left warp pipe. The Piranha Plant will still be there, but you should be able to enter the pipe, leading to World -1.

Another description is to stand on top of the exit pipe on world 1-2 and scroll the screen as far right as possible. Run to the right, then back to the left as far as the end of the pipe, then back again several times. The screen to reveal a blank space on the far right side of the bricks. Jump up and break the only second and third bricks from the right. Place Mario on the pipe with one foot off the edge and face him to the left. Press A to jump, and press Right while he is in the air. Do not break the remaining brick. The back of Mario's head should hit the brick and pull him in. Walk through the bricks to the right and immediately enter the first pipe on the left.