Statutory declarations are sworn statements by people who know of your relationship and believe it is legitimate. They can take pretty much any form as long as they are signed in front of a notary who then attests to and verifies the identity of the individual who signed the statement. These could be from friends, family members, neighbours, landlords, etc. The key is that two of them need to be notarized or "sworn" (btw, you can offer more if you want. I had 2 notarized and 6-7 more that were not).
General information you should ask the person to include in the statement
1. A description of how they know either of you, the applicant or sponsor, (typically the person they knew first)...examples I am a friend of XXX, I was a co-worker of XXX, I am the sister of XXX, etc
2. A description of how they became aware of your relationship, when they met the other partner, activities participated in together, etc
3. A statement to the effect that, in their judgement, you are in a real, loving, ongoing, genuine (whatever words are appropriate) relationship based on personal knowledge and experiences
This is what we did. We had a total of 4 notarized letters (2 done in Canada, 2 in Germany) and several others that weren't notarized. In Germany we got a "Ortsvorsteher" (municipal administrator) to notarize our letters and in Canada, a normal notary. We also signed the Declaration of Common-Law Union in front of the Ortsvorsteher. (Declaration of Common-Law Union is not mandatory, but it doesn't hurt, and it is a legal declaration that you are in a genuine relationship.)