It's actually pretty easy to make a costume for a dog, just as long as you keep a few basic concepts in mind.
Step 1: Know your limitations.
Step 2: Start with the basics.
Use some tracing paper or scrap fabric to make your pattern pieces. The first piece is a "coat" that covers the back and torso; second, a "cumberbund" that attaches the sides of the coat around the belly, and the "neckpiece or shirtfront" that attaches the sides of the coat around the front (and also down to the cumberbund if desired for your design). Use a tape measure to determine the proper dimensions of these pieces for your dog.
You will permanently attach one side of both the cumberbund and the shirtfront to a side of the coat. Pick a closure device to attach the other sides - I've always used velcro, but buttons or snaps are fine if that's what you have. If you have a full shirt front you'll want to add another closure connecting the bottom to the center front of the cumberbund.
Step 3: Design your costume around the basic structure.
Say you want to make a St. Patrick's Day costume. How about Oscar Wilde? OK, let's find a picture on Google. Now you know how to embellish your pattern pieces!
Henry the 8th? Same deal. Sure, the little hat might sell this outfit a little better, but add it to a group costume and the hat suddenly seems unimportant.
Historical figures aren't all you can make with this simple pattern. How about a New York Taxi? Driver's license optional. How about a subway car?
You can even dress him as a legendary dog - just add heads.
All of these costumes have the same basic structure.
Step 4: Bedazzle to your best ability.