Most of these things have been preserved before they get to a frame shop -- all I've seen already have a pin through their thorax, which may be conveniently stuck into something like a cork or foam center board. I suggest gluing the pin into the board, so it can't fall out.
Maybe there's no science to pinning the critter; maybe you just stick it through. Or, maybe there's a special way to do it. I don't know, so I would hesitate to take chances. If your butterfly -- or is it a moth? -- is not yet pinned, I suggest having it checked out, preserved if necessary, and properly pinned by a taxidermist.
I mounted some huge butterflies from Malayasia years ago and I think that they were shaped before I received them. I did have to fasten them to the backing board but I used some regular mounting pins for insects that I got at the local community college. Check out the Entomology Dept. of your local college, they should be able to guide you on attaching them to the board.
You can also go here to get information on mounting insects.
I agree with Framerguy. There are guidelines for mounting insects that should be followed to be acceptable to the Entomologist. Almost all of the insects that you will see in this business have been pinned and dried so you are limited in the freedom you have to be creative. You just hope that they have been pinned and dried correctly. They are very fragile and have to be protected from handling and insect damage. No matter how you choose to display the critters and addition of a small packet of moth crystals to the package will be of help. There is method of display for handling in schools called a Riker Mount that prevents damage by all but the most destructive child. The insect is placed on a bed of cotton (with great care) placed in a shadow box and glazed. It is not used by the scientific community because the insect cannot be examined by a binocular microscope to determine species and subspecies.
You may want to visit the internet and see what help that you can get. A vist to a local college or university would be a help to you. Many local public agencies such as mosquito abatement districts employ and Entomologist that my help you.
Be careful with the Riker Mount; if the insect is dry, you can inflict damage by placing it in the mount and placing the glass on top of it. In order to make the insect a bit more pliable you may want to put it over your steam kettle (for tea) and soften the tissues slightly. Be careful.