Here's a video that will help in getting it done - or written out instructions are below.
1. Begin with the square fully unfolded and flat.
2. Fold the square to the width of your jacket pocket. In most cases, straight down the middle should do it.
3. Fold up from the bottom to form a finished rectangle the same width as your pocket, and about half an inch longer.
4. Tuck the folded-up bottom edge into your jacket pocket and snug it down to the base. Adjust as needed to create a single smooth strip of visible cloth running across the top of the pocket. Note that some of you will need to clip the threads to open up the pocket. Use a small pair of sharp pointed scissors to clip a few threads to get this started and the pocket should open on up.
There should be no visible overlap between edges of the square in this fold. Everything should look like a single, solid piece. It appears as a single horizontal band of fabric (no more than ¼ inch) parallel to the top of the breast pocket.
Also, if you can iron/put a crease before tonight - would be great. If not, iron before the Saturday concert.
Looking forward to tonight.