Flattening 14-mil Mylar
Lay the mylar curled side up and heat it . This works for me I cut it to shape first then put it in my hot box (curing oven) at about 40 degrese C for a few hours .You can also lay a towel over it and use a medium to low heat Iron across the towel or a hair dryer can also work directly on the mylar .Have fun.
Terry.