Good question. If the fit is tight, and it is better if it is, it can be difficult. Last time I faced the situation I clamped the cord-lock in a vice to grip it and hold the hole open, and used a small screwdriver to stuff the cords through.

Cinch cords and cord locks are a handy addition to many pieces of clothing. I have bought inexpensive fleece pieces and threaded cinch cords through hems and ended up with a piece as good as one much more expensive. The tops are warmer if you can snug up the bottom hem and the pants benefit from adding cords to the feet.

I like the process of modifying my gear so I save money and get what I want. Adding cinch cords and locks, adding Velcro to sleeves, installing zippers for ventilation, and stitching on pockets are all easy modifications.