The ideal way to proofread the page proofs is to have the copy edited manuscript and the page proofs side by side and go over them word by word, punctuation mark by punctuation mark. The page proofs, being set in type that's smaller than the manuscript's, will not match up, page to page. So it's not just scanning straight across to see that the lines match, but...a diverging pair of lines. And "match" isn't the operative word, exactly, because there may still be uncaught errors in the original, as well as in the page proofs.
The trick for the writer proofreading her own work is to ignore the story and just stare at the surface of the page looking for problems. If the writer is a good fast reader (as I am) then the story keeps grabbing attention, pulling it away from the fact that, let's say, there's no space between the words "some" and "place," leading to "someplace" where it should read "some place." Or, of course, the reverse...a space where there should not be a space. A comma in the wrong place. No comma in the right place. Was it wrong on the copy-edited manuscript? (Yes or no? It makes a difference because writers are charged for making changes in proofs just because they realize the third sentence on page 263 is clumsy.) The writer then has to go back and re-read from wherever the story took hold, this time firmly attaching attention to the surface.
Inevitably, I find things that I wish I'd written differently. Unless there's going to be reader confusion, or the fix is very simple and won't affect the paragraph length, I leave them as evidence of my ineptness. In the course of this book (863 pages in manuscript, 490-something in pages) I found 17 errors. Ten were CE-transfer errors--the copy-edited manuscript had been correctly marked, but the marks didn't transfer (one of those produced a hilarious sentence by transposing a phrase to modify something else. Another one may have been, but since that was an early post-CE correction, it does not appear on the CE; I sent it by email and don't have a record of it. Two more were not caught in the CE stage (change was made by the CE, or by me, or both, but left a now-unnecessary word behind). Four more should have been caught (by me, by alpha readers) but were not, so those are definitely my fault. Three of those were easy one-word substitutions of the right name for a wrong name. The perfect fix for one would be to alter the first paragraph of a chapter, but that could easily result in a cascade of changes of pagination down through it (and the rest of the book, worst case.) Even with electronic typesetting, it's best not to make that kind of change this late...pushing whole chapters down the row leads to other unintended consequences. So I changed the "dateline" that orients readers to where something's happening. That will have to do. No line changes, no pagination changes, no problems with the rest of the organization.
Electronic typesetting has improved a lot since I first dealt with it, and I'm finding fewer errors. (Or maybe that last clause is the reality....I'm just finding fewer errors because I'm not as good at finding errors...) But I didn't see any compressed paragraphs of dialogue, where speeches by different speakers are bundled together in a confusing way, with maybe (or maybe not) a spare set of quote marks in the middle.
The process is, however, tedious...and slow...and I can't work at it more than an hour at a time without getting up to walk around and rest my eyes and brain. The moment I think I can work faster...I realize I missed something on the previous page and have to back up and do it again. It seems incredible, after the number of drafts, the number of re-readings, the amount of searching in earlier stages for just such errors...that they're still there, as if proof-reading were a whack-a-mole game and the same moles kept popping up.
In the course of proof-reading, most writers (me included) discover that some parts of the book still immediately "sing" and some parts...well, you can get very sick of a book after you've crawled slowly along its surface looking for the problems. The words "I am SICK of this book" came out of my mouth this evening, as I came back to the kitchen table to face the last home-stretch run at it. But it's important, and worth the effort, and now the effort is done. Bedtime.