This video is a really great guide if you want to restring your own doll. Sometimes all a doll needs is a hair tie wrapped around the elastic to extend its life span. But every doll gets to a point where the elastic is caput and needs new ones. Which is when you have to restring them completely versus the easy fix.
If this looks too intimidating, I do provide these services as well. But if you'd like to try it yourself, check out this video! :)
https://www.youtube.com/watch?v=kUYKracykSw