To get into Downward Dog, begin on the knees with the hands a few inches in front of the shoulders, fingers spread, middle fingers straight, perpendicular to the front of your mat. Feet should be about hip distance apart, or wider for very overweight yogis or pregnant yoginis (regardless of weight). Tuck the toes under and lift the hips toward the ceiling, keeping the knees bent at first. In the video I refer to this as "Half Downward Dog," and take the opportunity to stretch out the legs by bending and straightening the knees one at a time from this position. The upper body should be locked into a strong position-shoulder blades sliding down the back (upward toward the hips), elbows close to the body, chest dropped toward the floor. Make sure that the pressure is on the knuckles and fingertips and the shoulders are open and relaxed in order to alleviate any pressure on the wrists. If your wrists still hurt you can take this pose onto your fists instead. From the Half Downward Dog, gradually straighten the knees, moving the heels toward the floor and the sit bones closer to the ceiling. If you notice that you've made your pose a little too big, walk the feet in an inch or two to get the heels closer to the floor, but you're not going to want to bring them in too close. Draw energy up through the feet, firming the tops of the legs so that you feel like a triangle, with energy moving up toward the apex at your hips.
Here I have my lovely friend Marion Hodge demonstrating the pose for you. She has some slight tightness in the shoulders but does the pose perfectly despite this, whereas many people compensate for tightness in the shoulders by bending at the shoulders or waist. If this is your problem, begin with the Half Downward Dog and work toward getting the upper body aligned before attempting the straighten the legs. Thank you again for being my model, Marion!