Let me try to explain how I do it.
When I install the drawcord channels I sew the first one into place prior to stitching the insulation to the cover.
I fabricate my channel, insert the drawstring with the cordlock and then sandwich the assembly in between the two layers of covering fabric.
The channel should face in towards the middle of the quilt and the two layers of covering fabric should be wrong side out at this point.
Stitch three sides. Stitch down one side, along the sandwiched drawstring channel end and then up the other side. Be sure to keep the drawstring and cordlock clear of the seam while you are sewing.
Next stitch the insulation to one side of the cover while it is still wrong side out. I find it easier to do this with the cover on top and the insulation facing down.
Turn everything right side out. The insulation is now inside the "pocket" and your first drawcord channel is now exposed on one end. As a bonus the ends of the drawcord are stitched, neatly "turned in" and anchored on the corners.
Fold in one side of the cover so that it lays over the insulation and underneath the other side of the cover. Roll over and fold the other side of the cover to create a hem of sorts with the raw edge hidden. Sew through both sides of the covering and the insulation. It may be helpful to run two seams about 3/8" or 1/2" apart. One seam should be about 1/2" in and the other closer to the edge. This should give you a thinner and flatter area to stitch your last drawcord channel into place.
Fabricate your second drawcord channel. Make it just about 1" longer on both ends. Stitch it into place over the seam that you just stitched on the insulation / cover "sandwich". You'll be able to thread in the drawcord at this point and install your cordlock in the center "gap".
Sew / anchor the drawcord in place at the ends. Fold over the channel ends to form a hem and stitch through all of the layers and drawcord as you do this. Dependent upon your machine you may have to manually / hand advance through this thick seam.
As a result all raw edges and drawcord ends should be hidden. The insulation should be sewn into place on all four sides and both drawcords should gather and release smoothly.
It is your choice whether to put the first or second channel by the foot end or head end of your quilt.
Enjoy your quilt project and your finished quilt.
I hope this helped.