Much the same way you complete a writing or editorial project: one word at a time.
This past month and a half I've been eating an elephant, editing a 105,000-word novel for a repeat client.
I told him this past fall that my recent day job meant I couldn't edit the book on short notice with a quick turnaround. I could work on it only evenings and weekends.
He agreed and promised to send me the manuscript via installments as he finished polishing the text.
I got the first installment six weeks ago and dug into it right away, sending him the next a day a tracked changes version of the first chapter that showed the level of work I felt the manuscript required. He agreed.
The next evening I invested an hour in the project. And the evening after that. I gave myself a few days off when other events demanded my spare time. Otherwise I tried to accomplish something every day, if only a half-dozen pages.
Fifty editing sessions later, I just sent my client the full, final polished text.
Much as I would have liked to work on it for 10 days from 8 to 5, that wasn't possible. So I did what I could, one bite at a time.