"Cứ tự nhiên."
I find that going to google translate, putting the phrase in then pressing the audio button in the bottom right has pretty good results. Their text to speech engine is one of the best.
Is this "help yourself" as in, "help yourself to some food" (if you were staying round a friend's house)?
This. I've been translating as "help yourself" since that's clearly what Duolingo is asking for, but the phrase should most certainly be the translations you've given.
Doesn't this mean like "carry on like normal" or "make yourself at home"?
Yes, you can say "Cảm ơn" or "Dạ" or "Vâng" to respond.
- "Dạ" and "Vâng" are the same meaning, it means "Yes" :)
lol tu nhien can also be used in an irritated kind of manner as well it's interesting how many different connotations a word can hold