Super Mario Galaxy 2/Yoshi Star Galaxy

Saddle Up with Yoshi

 * First planet

Run down the path in front of you until you reach the area where you see a Yoshi egg that is trapped in a force field. Defeat the Magikoopa that appears when you close to the egg to deactivate the force field. Jump on the egg or spin to break it open and Yoshi will appear. You can mount Yoshi by either jumping onto him or spinning next to him. The Comet Medal is on top of a tall hill in this area.

Proceed towards the muddy path opposite of where you entered the area. In the next grassy area is a blue warp pad to a time trial. If you still have Yoshi, it's easy to let him eat the enemies to get three 1-Up mushrooms. Past the teleporter is a large shining fruit guarded by three Octoombas. When you've dealt with them, pull it out of the ground by grabbing on with Yoshi and walking backwards to make a Launch Star appear.

This is a small planet with a checkpoint flag, a bunch of crystals and a chance cube. Break the crystals until you find the Launch Star and move on to the next planet.
 * Second planet

In the third planet, you need to get to the top of a long series of ledges. You can use your pull technique to drag the spiky plants off the ground, or you jump around them and flutter kick back to the ledge behind them. From where you start you can get up onto the mountain by jumping up onto a ledge with a sign and using the jump pad near there, or you can find the jump pad at ground level, which skips a little bit of the mountain. When you get past the stone steps leading to an upper level, you can go counterclockwise around the mountain with a flutter jump to avoid taking the path past the Piranha Plant and some more spiky plants. At the top of the mountain you'll find three Octoombas and a pull flower. Use your tongue to grab the flower and jump towards a small planetoid, then use the Launch Star to fly to the fourth planet.
 * Third planet

Avoid or kill the Paragoomba and grab the checkpoint flag, then continue across the stone platforms. Use your pull technique to pull out platforms with green circles on them. You can defeat the spiders by hitting them with Yoshi's tongue and then walking into them while they're spinning. Use the pull flower to get up onto the second ledge, then jump onto the moving platform to get to the end of the walkway. Here you can pull out the wooden platform to get an easy 1-Up mushroom, then use your tongue to swing on the flower up to the moving platform. When the platform is at its peak, use the pull flower to swing over to the next platform that will take you up to the top. Swing on the flowers to reach the shiny Power Star. Headbutt the large crystal twice to release the Power Star and swing on the flowers again to reach it, by going past it then coming back to the right.
 * Fourth planet

Spiny Control
The planet is much the same as in the previous star, except there are a ton of Spinys and a Lakitu who will replace any Spinys you kill. Run down the path and free Yoshi from the egg. Jump up into the next area and you should see a Launch Star in a glass cage on the opposite side. You can stun Spinys with your tongue then eat them and spit them out at the glass cages to break them. There is another 1-Up mushroom off to the side of the area by the sign that tells you how to look around with the camera, and another one in a glass cage in the second grassy area where the Launch Star was in the last star. One of the glass cages also has a warp pad to the time trial. This time trial takes place on a planet full of Goombas, and your best bet is to quickly eat them all with Yoshi as fast as you can. When you're ready to go, break the cage the containing the Launch Star and fly off to the next planet.
 * First planet

The second area contains three glass cages on wooden posts. Destroy the one in the middle by grabbing a Spiny and shooting it from the stone platform on the side of the area. This frees the Luma, who will turn into a Launch Star for you. When launch, your path with cross another Launch Star, but it comes quickly, so start shaking the remote before you get to it to be taken to an optional planet. If you miss it or don't want to go there, you will go straight to the third planet.
 * Second planet

This is a small grassy planet with smaller planetoids circling it and floating platforms above with a Launch Star in the middle. Jump onto one of the moving planetoids then onto one of immobile dirt planetoids, then onto another moving planetoid and finally to the platforms. You won't fall off these platforms, so you can go underneath them to get a 1-Up mushroom. There is also a Life mushroom on one of the end platforms. You can get to the Launch Star by jumping onto the planetoid floating around the inside of the platforms then to the planetoid with the Launch Star, which will take you to the third planet.
 * Secret planet

Use your pull technique to pull out the platforms with red circles on them. Unlike the platforms with green circles, these are on a timer and will retract after a short period of time. You can get an easy 1-Up mushroom here on your way up to the next level. The next part is tricky. You need to pull out the wooden platform, then eat a Spiny and jump up onto the platform and over to where the Launch Star is. From there fire the Spiny at the glass cage to free the Launch Star and head to the boss planet.
 * Third planet

The fourth planet is the boss area. Use the pull flowers to get up onto the platform and the fight will begin. Giga Lakitu will throw Spinys at you. Eat the Spinys and spit them back out at the boss to damage him. Be careful of Giga Lakitu's lightning. It's best to nab the Spinys out of the air so you don't have to stun them once to swallow them. When the boss' cloud turns pink and looks electrified, he'll sweep across the platform, so avoid the yellow circle on the ground while he does that. He starts out throwing just a single Spiny, but after you hit him once and avoid his lightning, he'll start throwing two at a time, then four at once after that. The best way to defeat him is to grab the very first Spiny he throws while it's in the air, then immediately throw it back before he has a chance to move around. Once you hit Giga Lakitu three times, he will release a Power Star.
 * Giga Lakitu

Spiny Rainbow Romp (Prankster Comet)
The Prankster Comet in this galaxy requires you to use the Rainbow Star power-up to defeat all of the Spinys in the area before the time runs out. You get a Rainbow Star right in the beginning, which you can use to take out the Spinys on the way to the main area. Here you will find a bunch of Spinys scattered around with three more Rainbow Stars in the middle and another one over by the tall hill where the Comet Medal was. The trick is to keep up your speed and be near a Rainbow Star when the one you're using runs out. When you get to the main area, head to the right and take out as many Spinys as you can and come back to the center for another Rainbow Star as yours wears off. Continue clearing out the area counterclockwise from where you left off, making sure to get the Spinys behind the tall hill. When you're leaving that section of the area, grab the Rainbow Star there (even if yours hasn't worn off yet) to finish off the rest of them. If you can minimize the times where you have to completely come to a stop or run into mud, you can maintain very high speeds to complete the star.

Green Star 1
The first Green Star is above where the Comet Medal was. On top of the tall hill, backflip and spin to get it.

Green Star 2
The second Green Star is located in a small alcove in the volcano to the right side of the large fruit that turns into a Launch Star. You can either use Yoshi and flutter kick over to it, or just long jump to it.

Green Star 3
The final star is at the very end of the level where the normal Power Star was. Instead of breaking the crystal to release the Power Star, just continue along the pull flowers to the very end, then flutter kick up to the Green Star. If you don't quite make it, jump off Yoshi for some additional height with.