Yosef Karo

“ But where if he'll wait for hide that's made lishmah he will be delayed to do the mitzvah, then he should write it on available hide that is not made lishmah and perform the mitzvah immediately, while requesting hide that's made lishmah.
צריכה עיבוד לשמה לכתחלה. אבל במקום שאם ימתין לעור מעובד לשמה יתבטל מהמצוה יכתבנה על עור שאינו מעובד הנמצא ויקיים המצוה מיד בעוד שמבקש עור מעובד לשמה. ”

“ If written without scoring, it's invalid.
צריכה שרטוט. ואם כתבה בלא שרטוט -- פסולה.
9 He writes all its lines equal-length; one should not be longer than another. If one line is longer than the previous or the next, or shorter than the previous or next, it's kosher. As long as it's not arched, or like a tail, or round. ”