The bag need to have a knot and a passtrew hole if it does not you may need to make a knot or cut the bag. This will not work for all bags it has a inner diamiter of 10 mm (3/8in) and a outer diamiter of 38mm (1.5in).

so you may need to edit it for your bag.

Print 2 of the string holder parts.

I would recimend using rafts suports and 100% infill.

Use 2 small zipties to fasten them together tightly.

If you can use a plyer to grip the zip tie.  You want it to be snug.

This is recomende but compleatly optional cut off the end of the zip ties