Once I have gotten the paper off mosting washing and rinsing with water has gotton most of the adhesive off.
Plus useing a sharpened scrapper.
Most comes right off. Some need lots and lots of scraping and rubbing.
After it is dry go over it with a drywall sanding sponge to remove any roughness.
Then prime it with a latex stain blocker such as 123 or Sherwin William Pro Block.
But if the dw has been torn those areas need to be skimcoated with light weight drywall compound (mud) and sanded before primming.