Summarize:

Often times dog owners will choose a breed of dog which looks a lot like them.  This might be a good place to start when you are considering which breed of dog you want to dress as for cosplay.  If you already have the look of a certain breed, it will be a head start on your costume.  If you have curly hair you may want to try a Cocker Spaniel or a Poodle. People with straight hair can pull off the look of shiny, short haired dogs like Dobermans and Dalmatians. Thin people could dress as Greyhounds or Whippets. For a tough strong look, try a Bulldog or Pitbull. Cosplay costumes are different from your average halloween costume in that you don’t want to cover yourself in a big dog outfit.  Instead, you want to highlight that you, yourself, are the dog.  Wear some shorts or leggings and a shirt in the color of your breed. You could even choose a full, short skirt or tutu if you are dressing as a poodle or another fun, fluffy dog like a Pomeranian. Add little elements to your outfit to suggest you are a dog.  Try tufts of fur on the shoulders, or spots on your shirt. A popular option for getting an animal look is a Spirit Hood.  These are large, furry hats with long sides which drape down and end in mittens with paws prints on the underside.  This is a great start to a dog costume.  You can also find dog ears on a headband or make your own by gluing felt to a headband. If your hair is long enough to put in pig tails, you can make ears out of your own hair. You will want to focus on painting the nose and mouth of a dog by tracing around the tip of your nose in black, brown or pink paint, depending on your breed.  Also create the dog muzzle between your nose and upper lip using a lighter color.  Here you can add some whiskers.  For a playful look, paint a red or pink tongue over one side of your bottom lip and extending past it to look like your tongue is hanging out.  Outline the tongue with a thin black line. Spotted dogs can paint a big spot around one eye. If you have a Spirit Hood, your front paws are already covered.  If not, it is simple enough to use a pair of mittens, which you can wear on their own or decorate with furry material or felt.  Boots are very common in cosplay.  Find a pair of boots which matches the look of your breed.  Choose a nice sleek, tall boot for a short haired dog. Find a fluffy boot like furry Uggs for a long-haired look. No dog costume is complete without a tail.  Depending on the breed you chose, you may want and thin tail or a fluffy tail.  You can achieve these looks using felt or yarn in the appropriate colors.  For a fluffy tail, cut 10 pieces of yarn 5 inches long and tie them together in the middle to make a sort of tassel.  Make several of these tassels.  Using the string tied around the middle of the tassel, tie these tassels to a longer string which is a little longer than the length of the tail.  Tie this to a belt or use more yarn to wrap around your waist and tie the tail on.  Make a tail out of felt or another material of your choice.   Cut the material five inches wide and the length you would like for your tail.  Roll the material starting with the end of the tail, which should come together at a point, ending at the top, which should be left open.  Sew the edges together to create a long cone shape.  Stuff the tail with cotton stuffing and place a flexible wire in the center if you would like to be able to adjust the shape of the tail.  Sew the top together and sew an additional loop to the top to thread a belt through. The accessories are what will really give your costume personality.  Think of different accessories and props you can add to your costume.  You probably want a collar or at least a necklace of some kind. Wear dog tags and have your name etched into the tag at the pet store. Maybe instead you want to get creative and use a bow tie as a collar to class up your dog costume. Some props you can consider would be a big stick or a dog bone.
Choose your dog breed. Wear clothes which give the impression of your dog breed, but still look cool. Get some dog ears. Paint your face to look like a dog. Change your hands and feet into paws. Tie it off with a tail. Accessorize to make your costume stand out.