To get to World 8 from World 5, do the following. First, head to the house directly after the first castle on World 5. Once there, go up six flights of stairs. Next to the seventh flight of stairs, there will be three bricks. Jump on top of them and jump straight up to hit an invisible block. Then, get on top of that block, jump straight up again, and hit another block. This one will unleash a vine that you can climb up. Once atop the vine, there will be a door. Go through the door and get on the nearby colorful platform. The platform will bring you to another door. Go through that door and find the red flag with a skull on it (as opposed to a black flag). Jump on the flag so it comes down, thereby completing the stage. Then, go to the cannon it shows and you will be brought to world.