stretching wall to wall carpet

[jjobean]

how much do you allow for stretching the carpet ? if i'm doing a 12x10 room I'm thinking i attach the 12ft side to the nails then stretch it sooo- do i cut the 10ft side a little shorter and stretch it over or do i start with 10feet and trim off the excess. also does it get stretched in the other direction or just one way. tks