You select 32 3rd string QBs in CFM. Make sure they are rated low. Turn auto-start off. This will make the CPU battle it on themselves. Now the setback is that the comp controls trades, releases, depth chart, and free agency so the only thing you can really do is watch the action.
When I tried it, the Pats kept releasing Edelman which was annoying me so this is what I did. Before I selected 32 players, I went to the menu screen, I ranked Edelman low, released him, and then resigned him to a new contract. Then I rated him back back. It's tedious work. but I did it.