Star 1 - Chip Off Whomp's Block
To get this star, travel to the top of the fortress and defeat the giant Whomp. Just jump on his bandaged back when he tries to crush you three times, and the star's yours.
Star 2 - To The Top Of The Fortress
Just go to where you fought the big Whomp, and instead of him, there will be a tower there. Climb it using the moving blocks, and you'll find the star at the top.
Star 3 - Shoot Into The Blue Wild
To get this star, you must first speak to the pink bob-omb and get him to prepare the cannons for you. Having done that, all you need to do is shoot yourself onto the platform to the left--the one under the flag pole. Once you get there (it may take a few tries if you're not used to the cannon), climb down the pole and the star's right there.
Star 4 - Red Coins On The Floating Isles
Find the 8 red coins and a star will appear somewhere on the field.
Star 5 - Fall Onto The Cage Island
First, climb up the tree near the start of the level. Once you do so, an owl will come out and offer you a ride. Jump into his claws and steer him towards the cage that holds the star up there in the sky. Drop yourself in and claim your prize.
Star 6 - Blast Away The Wall
Go to the cannon again, but this time aim to the right. You want to hit the side of the brown wall near the top. If you get this right, it will shatter revealing a star. Climb up to the wall, slide down to the star, and you've got it.
Star 7 - Get 100 Coins
As with every level in the game, getting 100 coins automatically nets you a star.