Go to the organizations web site and use the listed email contact. They will put you in touch with a mentor to help you. It is only one fee for a given line even if you have to supply additional information. I.e. it is not open ended charges.
You only have to prove back to someone already proved in their database. For example if your 2nd cousin was accepted, you only have to provide proof back to your mutual great-grandfather.
If there were no birth certificates at the time, they do accept other reasonable documentation including censuses, wills, perhaps obituaries of the time if you can prove the newspaper.
So get in touch by the email and get that mentor.