loads/buckefuls of salt .. Everylast bit of stuff needs scraped off the hide. Salt salt salt the hide. I always trim it all around so no excess edges. every 3 days shake off the salt and scrape the hide , you have to be careful to not scrap a hole in it. I use this half moon shape scraping tool.

Roll it up tight for a day. then unroll , I rubbed it tightly over a saw horse in all directions. Again salt it alot ! like 2-3 things of salt ( thank god they are cheap LOL)
Keep repeating process. Depends in the weather.. but usually takes me about a month of this until I am satisfied its supple enough.

Tons work to be honest, But I think its worth it.