Before this thread ends up with a lot of basic questions, my first advice for anyone that gets divorced prior to the 90day window for filing a joint I-751, is to read the I-751 instructions on the USCIS website! Seriously... Go through the instructions, and highlight anything that pertains to your situation with a marker... Then go and collect those items!
If you're not yet divorced, do whatever you can to collect all the required evidence - Your spouse may try to destroy some of it!
You can file I-751 as soon as you're divorce is final (ie, you have the Divorce Decree in your hands!)
Make sure that you check the USCIS website right before you mail everything off to double check that you've got the correct filing fees (these seem to change on an annual basis), and the correct mailing address!
I don't think it's required, but having a cover letter that lists everything that you're sending, in my opinion, isn't a bad idea!
For taxes filed jointly, you want to send a transcript from the IRS - You can call up the IRS and go through an automated call to request this.
Sometimes, friends can take their sweet time to write affidavits if they're just plain busy with their own life. Better to ask for these sooner rather than later!
When I sent photos, I wrote some info on the back explaining when / where they were taken.... Saves the Immigration Officer trying to figure out the details!
I've not completed my own I-751 divorce waiver process yet, and I'm not legal expert, but the above is simply my $0.02 to people in the same situation!