Normally this should be handled as a simple character reference type of letter. Depending on what you're using the resulting affidavits for (visa application, applying for benefits, etc.) you may want to ask that your family and friends have their signatures notarized (i.e. to prove that they are who they say they are and that you didn't just write the letters yourself).
But it should be sufficient for them to write a letter, saying who they are (in relationship to you), how long they have known you (and your spouse) and any details they think would be relevant in support of your proving the bona fides of your marriage. (Example: indicate how often they have entertained both of you at their homes, did they attend your wedding? how often have they visited you at your home, etc.)