Feed the cotter pin through the drawstring casing.
The casing will vary on each garment. In this case the drawstring casing is visible on the exterior of the garment, but it may be on the inside of your garment.
Insert the cotter pin—still holding the drawstring—into the drawstring hold.
In this case, the drawstring hold is a small hole in the garment. The drawstring hold is just the place where the drawstring will be sewn to the garment. This may vary from garment to garment.
Pinch the drawstring inside of the drawstring hold.
While holding the drawstring, pull the cotter pin out, sliding the drawstring out through the spilt in the pin.