My best friend's daughter was turning three, and because whenever I am over, Diego is with me, she made sure to invite Diego to her birthday party. Unfortunately for him, the theme was 'Mermaids and Pirate Ships' and she personally requested he be the only male to dress up as a mermaid. I couldn't really let her down as it has been quite a hard road to get her to like his presence in her house. That is when this costume was originally made, but it makes for a perfect dress up for Halloween as well. I also have a matching mermaid tail in case we wanted to do a couples outfit.
I purchased some cheap material from Pete's Emporium, and although the material from here is questionable in quality, it is perfect for little tasks like this which won't be worn very much. I found the material to be of the perfect pattern that no matter how terribly I whipped up the costume, it would still look somewhat mermaid'y. From there, you just need some cardboard (I used an old beer box. Recycling!) and something to attach it to your dog. I used Diego's Linden No-Pull Harness from Wolves of Wellington, but you could also just attach it to their collar.
The first step is to cut out the fin shape in your cardboard. This is going to give you the main shape of the outfit so depending how you want it, this is the important part. I drew one half of the fin onto a folded piece of cardboard and cut this out. This made it so that both sides of the fin would be even. If you trust your drawing and cutting skills, you can just draw the fin straight onto cardboard and cut.
Once this was dry I placed the cardboard fin onto where I would like it to sit on Diego and measured roughly to where his harness sits. You will want to go a few centimetres over this point and then cut. If your tail is connecting to a collar (which I am not sure how it will turn out, but I can't see it going too wrong) you obviously will need to have more material than if you have a harness. I personally wish I put Diego's fin slightly further down so that it sat a bit better with his tail so make sure you really test where the fin sits before cutting. This is also the point when you will cut the sides judging by how big your dog is, how big the fin is and if you are attaching to a harness or collar this will change. I cut mine so that it was just the correct size of Diego's body, but if you had a larger fin you may want to make it so it drapes down the dogs sides.
Where you cut the material you will now want to make a loop in the material that the harness or collar can slip into. If you have ever purchased a dog bandana it is similar to the concept of those bandana's that just slip over your dogs collar. I sewed my loop down, very poorly might I add, but once again depending how permanent you want this to be, you can use a hot glue gun. If you want you can also sew the edges to make a nicer edge, but I left mine jagged.