When I sat down to write this it struck me just how many things there are going on in this pose. Whilst there may be specific things you should and shouldn't do to keep a pose safe (refer back to some of my points about Chaturanga where you'll see I was very specific about a couple of alignment points) you don't need to worry if you can't take on all of the points about a pose in one go. Generally I call out the most important points or those points which have the most significant impact on the pose or I will comment in class on what I see being done (or not done), so don't worry about trying to get every single element nailed in one go....patience (young grasshopper).
Hands are about shoulder width apart and feet are about hip width apart. Note the use of 'about', start shoulder/hip and then move a little bit if slightly wider or narrower feels better in your body.
Aim for your wrist creases aligned with the top of your mat, spread your fingers and pay particular attention to pressing down through your thumb and first finger which avoids excessive pressure in the wrists and through the outer side of the forearms and up into the neck and traps. If you find Down Dog is too much for your wrists you can try making fists with your hands and resting on the knuckles (inside of wrists facing each other), try using a yoga wedge or drop to your forearms. You can also try building up the amount of time you spend on your wrists in this pose and your wrists should get (more) used to the pose.
Heels are moving towards the floor. It doesn't matter if you have been doing yoga for five years and they aren't down yet - it's not the be all and end all. If you are tempted to set this is a your number one aim for the pose you might find it doesn't feel particularly nice on your spine. Focus on the following tips and when your heels are ready to come down they will.
Knees bent a little or a lot. If you have tight hamstrings you'll be more comfortable with a deeper bend. Ditto if you find your lower back is rounding, a bend in the knees will help straighten the spine.
Neck in line with the spine; don't be tempted to drop the head too much (and yes, before you point it out, I am dropping my head too much in this picture). Take your head from side to side and think 'ears to biceps' - if only I'd done that before taking this photo!
Draw the lower belly in towards the spine. A strong core (not just the abdominals) can help take some weight off shoulders and wrists. This is true for a number of poses which require upper body strength e.g. crow, headstand and plank, engaging your core helps to spread the effort/weight and makes you 'lighter'.
Firm your your shoulder blades and draw them towards your tailbone. Thinking about this feeling will help avoid scrunching the top of the shoulder blades and around the neck and will feel more comfortable.
Try to imagine that your thumbs want to draw in towards each other, this slightly rotates your forearms in towards each other. At the same time, try and rotate your upper arms away from each other (externally rotate). See if it helps to imagine that you are tucking your armpits under. It may sound tricky to rotate the upper arms out and the lower arms out but it's not a massive movement you're aiming for and anatomically it's fine. It'll help keep shoulders away from ears and create space across the upper back. This tip can be difficult to think about let alone practice so don't worry if you think you haven't got it yet.