1) first thing first make sure you actually have a contour colour and not a bronzer. bronzer is designed to warm up the face. a contour is a ashy done cool colour with mimics your natural shadows.
start off by applying a line which goes from the above your ears to the half way point towards the corner of your mouth. stopping at the most hollows of the cheeks (which is about the half way mark)
you would depth to that line by making it thicker near the ears and tapered off as you are coming towards the hollows of the cheeks. Keep majority of the concentrated colour near the ears and near the bottom of the contour line line the picture below.
the above picture i know is not drawn to accuracy but it should give an idea on the location of the most pigmentation of the product.
a good contour is when there is no line. so you do need to make sure that you buff out the edges making it seamless blend into the skin, do not forget the ears and the hair line when blending also.