Nose, ears, eyes, in that order. Why wind decides everything else.
In that order. Understand the ranking and most of field craft explains itself.
The nose beats you. Under the right conditions a fallow can pick up human scent from several hundred metres. There is no clothing, no spray and no technique that defeats a bad wind. You either hunt into it or you don't hunt that patch.
The ears are close behind. They rotate independently and pick up frequencies well beyond human range. A snapped twig or a crinkling jacket carries far further than you think.
The eyes are the weak link, with a catch. Eyes set on the sides of the head give them close to 360 degrees of vision. They are outstanding at catching movement and comparatively poor at resolving a stationary shape. Freeze and you often disappear. Shift your weight and you're gone.
Deer see well into blue and ultraviolet and poorly into red and orange. Nearly every laundry detergent contains optical brighteners, which are engineered to fluoresce under UV so your whites look whiter. To a deer at last light, a freshly washed white shirt reads a bit like a hi-vis vest. It's the real reason experienced hunters wear dull, unwashed-looking gear, and the same quirk is why blaze orange keeps you visible to other people without lighting you up to the animal.
Most active at first and last light. A high proportion of rod cells in the eye gives them strong low-light vision, which is exactly when you want to be out.
Fallow are intensely social, sometimes in mobs of a hundred or more. Every animal is another set of ears. Where there's one there are usually twenty, and you have to beat all of them.
A sharp, explosive single bark. Once one animal barks, the mob looks to the lead doe before deciding. If she settles, they often settle.
Glands between the hooves leave a scent trail as they walk. Other deer follow them, and established trails get used again and again, year after year.