Well you can either make your own, or you can request a signature to be made from people who make signatures here on TBT. Although, there's often a fee so it's best to save up some TBT. I can't remember where you can find the section of the forums where people make signatures, but mine was done by a kind user named Mayor Kaleigh. I think it costed about 60 or 50tbt? i'm not sure though, it was a while ago. Good luck on getting a sig!
You can easily make one from using graphic/picture editing software, and of course skill and practice, or you can easily get one on here from the Museum Shop and the Museum sections of the forum. There's plenty of people looking to sell and practice making signatures, so I'm sure you can easily get one without much work!
its much simpler if you get them from the museum shop than making them your own. coz i know some users here make their own AC villager sigs and tbh, it kinda looks meh compared to the one you can get from the museum shop.