I hope this still applies! My sister went thru this about 6 years ago. So typically you'll have to have been together for a significant amount of time (nothing crazy but not like 1 year either). I know my sis and her bf were 7 years strong at the time. For proof of relationship they used pictures, email exchanges, bills (he owns a house and she helped with electrical and water so her name was on the statements). Thay also worked together so they asked their work to write a notarized letter. If they still allow this -which i really hope for your sake they do (sorry if i got your hopes up for nothing) - they will tell you what works as "documentation" of your relationship. I really think your best bet is contacting the consulate.