I need to create a form on my website like this:

http://www.buildasign.com/CustomBanners

I'm compiling ideas, but not really sure where to start. Any advice is greatly appreciated

Which form? There are several on that page.

Something like this:

http://www.buildasign.com/PowerDesign.a … p;CID=1235

For customers to create and upload their own designs.

I would give this a try. I used it on one of my sites to build a map designing app. It's pretty awesome and fully customizable.
http://fabricjs.com/
Click and alter the things on that page and you can see just a little of what this script is capable of.
Here's the kitchen sink.
http://fabricjs.com/kitchensink/

Now that is cool! I also found after doing some reading about fabricjs that there is also this available:
http://fancyproductdesigner.com/jquery-plugin/
Is is based on fabricjs.

Fantastic link Eric!!