Super Meat Boy no-death achievements

I have Super Meat Boy for Steam and there are the no-death achievements, like Wood Boy, that expect you to complete a world without dying.

What exactly do I have to do to get one of these achievements? Do I have to run through all levels in a world in order, possibly including level -1 and the Boss? Or can I do the levels out of order assuming I don't die on any attempts?

Here a video guide on game trailers.

http://www.gametrailers.com/user-movie/super-meat-boy-wood-boy-achievement/352916

You have to complete 20 levels in a chapter without dying. You can complete the levels in any order as well.

To get the no-death achievement of a chapter, you have to complete the 20 main levels without dying. There are separate achievements for light world and dark world.

• you can freely switch between characters, but Steve and Meat Ninja don't count for these achievements,
• you can do the levels in any order,
• you can switch between levels as long as you don't die (so you can quit a level just before dying).

You can take some decisions to make the achievement easier:

• go from the hardest level to the easiest level,
• choose the best character for each level.

It's better to unlock all characters before trying to do the no-death achievements.

You can now find a lot of guides in dedicated websites or in the game's steam page.