In my case, when I upgraded from Cat5 to Cat6, I taped on to the existing 5 and used it to pull the 6 through the attic and down the wall. In the attic, you usually use a string, with a weight of some kind attached (if it's a new install), to drop down the wall seal plate, which you've drilled a hole into, down to the hole you've already carved out for your cable box near the baseboard area.