Flatten it. Trim the bottom of the bag along the seam, just above it. You want as much usable mylar as possible to make your notebook.
After flattening and trimming find the air lock. On some bags it’s clearly visible and other’s it’s harder to find. I noticed that starbucks and Pete’s hide theirs in their logos. Keeping with the theme of keeping as much usable mylar as possible trim straight up from the bottom along one side of the airlock and them up the other side, you’ll be left with a strip of mylar about 1 inch wide with the airlock in it. Discard it.
Use a damp sponge wipe down the mylar. Then use a spray cleaner to continue to clean it. It may take several cleanings to get it clean. Don’t leave any of the coffee oils on the mylar as it will stain the paper when stitched in.