The plastic bag is call "Flyer", there's small, medium and large. For clothes, most likely will need a medium or large flyer. Just go to the Poslaju counter and ask for flyer. If you're going to small post office, they may not have flyers so you have to wrap your clothes up in whatever paper you want.

Fill up the orange color Poslaju form properly, make sure you declare what is inside, and sign it. Within peninsula Malaysia will cost RM5.95 cheapest, if your clothes is heavier than 0.5kg then will be more expensive.