1.Go to a website that has animal crossing icons. http://ouka.kuronowish.com/rehoeass2/ is the one I use.
2.Then, you save a picture for the background.
3.Then, save any pictures, icons, or other things you want in the signature. I mean things outside the picture, like Alice's knights or whatever they are.
4.Then you go to photoshop or pixlr. Pixlr link: https://pixlr.com/editor/
5.Then you save your work.
6. Then, you go to TBT and go to Settings - Edit Signature
7. Attach the picture, and maybe a colorful message. I can get you the color code if you want.
Sometimes they do it for free, but mostly for the right price. So long as you follow their rules in your request, it should be a decently quick process. If you wish to create your own, please follow the guidelines.