I am trying to find a book I read in the late 80s, possibly early 90s. It is a fantasy novel, although there is a bit of a post-apocalyptic feel to it. It was set in a desert. The main character is a female apprentice magic user. The magic system used involves affixing metal shapes to your body (perhaps moon or scarab shaped), which allows you to handle a higher level of magic. The metal shapes are applied with the help of a higher ranking mage. The magic users also use spell components carried in a pouch on their belt. This is less of a Harry Potter kind of magic, and more of a recipe that creates an outcome magic. At the end, the main female character needed a power boost in a hurry and tries to attach a stolen metal piece to her arm in order to become more powerful, but one of the pieces is affixed incorrectly, and it limits her ability.