I’ve done a lot of couching on art quilts over the years. There are several different kinds of couching feet for domestic machines. Some of them have a groove that runs front to back on the underside of the foot that the yarn or cording you are couching down run through. These work best if you use either a zigzag stitch or a decorative stitch that goes from side to side over the material being couched down. You can also use a zigzag foot for that type of couching, but it can be a pain to keep the material being couched down in the center and it can sometimes get stuck under the foot as well.
Other couching feet have an opening that guides the thread down and through the foot right below the needle and you then use a straight stitch that goes through the yarn or cording that you’re couching down. There are also couching feet that work like these ones that can be used with longarm and midarm machines for couching while FMQ. The main thing to remember with this type of foot is that you have to make sure that the yarn or cording fits the hole properly, if it’s too loose in the hole, it can easily get moved out of place and your straight stitches won’t catch it properly. You also have to be mindful that you keep the yarn or cording loose just ahead of the foot or it can get pulled out of the way and your top thread won’t catch it properly.
I’ve couched all kinds of things to my art quilts doing the zigzag type couching with a foot that has a groove in it. These materials iinclude embroidery floss, yarns, rattail cording, charging cords from old electronic devices and thin sticks. They just need to be small enough to pass beneath the foot. On my Sweet Sixteen I've done a fair amount of free motion couching using the type of foot where you pierce the item being couched and in that case have mainly used yarns, being careful to pick ones that fill the hole in the foot without being too tight.
Rob