Ok, here is the way I make my collars for pluckers/mutilators.
Step one, get all equipment ready (gauze pads, medical tape, scissors, toilet paper tube/paper towel tube). (measuring tape is for size comparison)
Step two: Cut a section of the cardboard tube (wide enough to stretch the neck out a bit):
You may need to cut it shorter so there is only a very little bit of overlap when placed on the neck.
Step three: Wrap the cardboard piece in the gauze (make sure it is stretched tight and use the whole piece of gauze for maximum comfort).
Step four: Make sure the end piece of the gauze is on the outside of the collar so it doesn't rub on the neck, then tape it with the medical tape.
Step five: Cut one end (the end that will be closest to the skin) rounded so it doesn't have corners that will rub too much.
Step six: Add a bit of tape to the other end (the end that will go over and tape the whole collar on).
Step seven: Wrap it around the bird's neck. Make sure the wings are able to move, but the neck has little give so there is little room to reach around to the back of the body.