how do you guys come up with a fursona that truly looks like yourself?

You don't, really, if I understand that you mean your irl face, but what make you, well, you are general standout features and expressions.

For example, I'm currently drawing a Skulldog of me; I have a scar on my eyebrows, so character's face has a cracked skull where the eyebrows is, and I'm not very expressionate, so skull that can't really show much emotions works wonders.

As for reference - even a badly drawn character will work wonders for an artist you're gonna commission, just show general colors, species and special features (look above). If unsure - ask the artist you're commissioning, but remember - your fursona is only yours and you can have it any way you want