I have recently tried this because I was likewise curious. First of all, it is critical that there be no commercial yeast in the dough. Otherwise, the commercial yeast might dominate. Second, the dough should be reasonably fresh to ensure there are enough healthy yeast and bacteria to be revived. Third, there are likely other ingredients in the dough that may not be desirable such as oils, sugars, etc. But the reviving process will likely dilute these to the point that they are no longer a factor.
What I did was mix in enough water so that I had a batter-like consistency. Then I added equal parts by weight of water and flour at regular intervals until you have a bubbling, happy culture. I did this about every 4 hours for a day, throwing out some of the excess each time. The dough I was using was very fresh. It was easy.