Bring the straps up through the holes on the bottom of the seat, one on each side. The metal plate on the end should pull in flat and snug against the plastic. Slide the buckle pieces back on, making sure they're facing the right direction to snap in. Slide the chest clip back onto both straps.
Place the ends of the straps through the slots you want the harness at (at or below the child's shoulders for rear-facing, at or above the shoulder's for forward facing), making sure that they don't twist. Pull them down low behind the seat.
The adjuster part of the strap has a kind of Y shaped metal plate - pull it up to meet the ends of your straps, making sure that it
doesn't twist, either.
There are loops at the end of the harness straps. Feed the curved part of the Y through the loop of the strap that's on the same side of the seat. Pull it all the way through, so that the fatter part on the ends of the metal plate are exposed and curve around a bit beyond the strap so that it doesn't slip back off. Do the same with the other side.
Now adjust the harness to fit your child. When it's tightened properly, you should not be able to pinch up any slack, lengthwise, at the hips or shoulders, and it should lie in a straight line down the front, with no drooping. Also adjust the chest clip - it should lie even with your child's armpits - not up in the neck, and not over the belly, where it can't do it's job properly. (It's job is to hold the straps over the center of the shoulders...if it's over the belly, they're likely to slip down off of the shoulders in an accident or sudden stop and also poses the danger of internal injuries.)
One other thing - car seat harnesses should never be submerged in water, as it can damage and weaken the webbing. They should only cleaned with a damp rag. If you put your harness in water when you washed the rest of the seat, I'd strongly recommend contacting Cosco (contact info here
) to order a replacement harness.