Bring Luigi to World 1-3. When you reach the first pulley lift, stand on the left platform and let it go all the way down until it breaks off, then quickly jump to the small platform on your left. Next, do a running jump (hold the B button) to about where the right platform fell off, but a little lower. If you jumped at full speed, you'll touch an invisible platform, but it's a little tricky to land on it without sliding off.