To get three stars on a level you need to clear the stage, collect every Zap that's hidden away, and beat a time the developers have set.
If you stick to the beaten track, you won't find all of the bolts you need to get three stars.
If you die and respawn, there's usually a power-up station nearby so you can get tooled up again.
You lose cogs if you die. Try and stay alive for a better reward at the end of the level.
Tap the screen again after your first jump as it will propel you higher into the sky, allowing you to latch onto cord-swing nodes, or just reach a new platform.
The magnetic power-up allows Cordy to stick to the walls he's running along, even if that sends him upside-down! You can't just climb straight up a vertical wall, however. To do this you'll need to take a run-up along a surface that curls into the straight. If there are gaps in the floor preventing you from doing this, have a hunt around and see if you can find any blocks to fill it.
Cordy's rocket-fueled friend uses inertia physics, so be very careful when steering him upwards. A tap of the screen will add some boost to the rocket, but just make sure you don't collide with any enemies or obstacles when doing so. Slowly does it!
To use your block-smashing friend, grab the power-up and then position yourself new the glowing blocks. A tap of the action button will cause him to rip through the blocks. You'll sometimes find cog currency locked inside these, so make sure you've grabbed them all before moving on.