Cut a 1 1/4-inch-wide strip of poster board 1 inch longer than the neck size of the shirt.
Shape the strip into a ring overlapping the ends by 1 inch.
Stick a 3/4-inch square of hook-and-loop tape between the overlapping ends, to hold them together. This is the collar band.
Turn up the shirt collar, and place the band around it with the opening in the back. Fold the collar down over the band, so that there is a space between the ends of the collar about 1 inch wide.
Tack the corners of the collar to the shirt, with a needle and thread, to hold them down. Do this by sewing a few small stitches close together.
Wear the shirt by removing the collar, putting on the shirt and buttoning it up. Insert the ends of the collar into the opening in the front, and pull them around under the collar to the back. Fasten the hook-and-loop tape, and smooth the collar over the band.