First, you are on the right track. USCIS will not tell you to file the I-751 and will not remind you. You must take the initiative to file within 90 days prior to the expiration of the LPR date. Both of you must sign the I-751. Check it over carefully for errors and do not forget the filign fee. Yes, send copies of tax returns, joint bills, insurance, birth certs for children together, lots of photos in different locations, different occasions, with different people, at least ten or twelve, joint memberships like Sam's Club, affidavits from both sides of the family, mail sent to you with both names on the envelopes, proof of involvement with stepchildren if any. It takes about eight months to get the new card so hold on tight to the receipt you get. That receipt cannot be replaced if lost. It is your only evidence of continued lawful status and you will need it if you travel while waiting for the new card. Some local CIS offices will stamp your passport to extend your green card upon showing them the I-751 receipt but many will not. You cannot send enough evidence, so be generous with your filing. Most people do not get a request for evidence so do not worry. Check on your case regularly starting at the four month mark at www.uscis.gov using the receipt number. You are eligible for filing for naturalization at 90 days prior to the THREE year mark, so do not be surprised if you re able to file for naturalization almost immediately after getting the new card.