Choose model picture (e.g. photo) and while you draw watch your drawing through the phone or tablet (that need to have camera). Model will be shown semi-transparent on the top of your ongoing drawing helping you to outline the contours. It is best if you can attach phone to some kind of support stand so that it stays still and both hands are free. This is not cheating but a new way of working and learning as well.
Free trial version for Android phones and tablets is available from Google Play.