doubling yarn and garter ridges

[sberb]

hello - i'm pretty new to knitting and after too many scarves i'm moving on to what i hope is a fairly simple felted box - i'm already feeling overwhelmed though! in the gauge instructions it says that the yarn is to doubled throughout the pattern. how exactly is that done? is the yarn folded in half across itself or cut?? cripes.

also, it says "CO12 sts. Working in garter stitch, knit 4 ridges." does this just mean that as the knitted rows make ridges on their own? what exactly does it mean by 'ridge'?

thanks for any help,

sara