The Cat Suit is required for this cheat, so pick one up in World 1-1 Super Bell Hill and hold it in your inventory or wear it through the level. Near the end of the underground portion of the level is a pipe that splits into two paths, one with a loop with coins in it.

On the far side are three long ? blocks that are invisible against the rear wall. Use these to reach the upper path with a lone Koopa patrolling it. Above this Koopa is a hard-to-spot gap in the ceiling. Run up the rear wall to find a red warp pipe. This will warp you to World 2.