In 8-2 you will see orange tiles (stuff you can walk on and jump through)next to it a fake wall.(the ground level will be raised over it)walk over it then go through it.you will see you are under the orange tiles.keep walking left to go through another fake wall. You will see a tube.go down it and when you are keep walking and find the red flag.then you unlock 8-7
Right away when you enter the ghost house of world 4 go left into a fake wall then jump to reveal a hidden block with a star. Then you can run through all the boos and little mousers, making the first part a lot easier.