@anamarie Hi ana, StatDecs are commonly used in Australia, if you are lodging your visa onshore (in Australia) then you can use this link as reference and have it signed by a barrister, doctor etc (people allowed to sign Stat Decs as per Commonwealth law)
https://www.ag.gov.au/Publications/Statutory-declarations/Documents/CommonwealthStatutorydeclarationform.pdf
If you are lodging offshore (any other country except Australia), then a written statement detailing your relationship would do. Just make sure that you can prove your relationship. Have it signed by yourself, if you want you can have it notarized as well (you won’t really need to have it notarized).
Have other witnesses write down a statement about you and your spouse’s relationship and let them sign it, you can use that as well.