OK, Anacrusis, you can take the tin hat off now. I actually agree with you.
Gwu, sight singing is a smallish part of a smallish part of the exam. If everything else is OK, you'll get decent marks for the aural part of the exam even if the sight singing goes totally to pot. Don't forget, the aural marks are based on an overall impression of your responses rather than being split up into marks for the individual questions.
I agree that intervals are the way I (and many singers I know) sight-read. And
if you're in practice, they are a very effective method.
But with only a few days to go, I think it's not worth spending vast amounts of time in getting in practice for this - getting your scales and pieces beautifully polished is a much better use of your time.
Assuming you want to get the hang of sight-singing for the future so it's less of an upheaval for your next exam (grade 7 or 8??) the best bet is to do some singing on a regular basis. I'm a great advocate of joining a choir as a way of improving one's confidence at actually singing, and at sight-singing in particular. Advantages - you're not alone when you're trying to read a new line of music, you get feedback instantly, and if you choose your choir carefully, you get a great night out as well.
A useful resource for practising sight-singing on your own is the
Choral Public Domain Library. You'll find thousands of works there, many of which will have both a PDF file of the music and a midi file. Choose a work that has both. Open both the midi and the PDF, and listen to the former while reading your chosen line from the latter. Do the first one just reading, not singing, then do another one by going through it twice, just reading the first time then singing along the second time. DO NOT SING THE WORDS, JUST "LA" or "YAH" TO THE NOTES. And then do another one but sing from the outset. And then finally choose one, sing it and then play it afterwards while looking at the score to check how you did.
I also use CPDL for practising the sing-back bit of the Grade 6, 7 & 8 aural tests. Choose a work with the right combination of voices (2 parts for grades 6 & 7, 3 parts for Grade 8). Open the PDF and play the midi. Listen to the top (Grade 6) or bottom (Grade 7 & 8) while following that line on the score. Then sing it back from the score. Then choose another work, open the midi and PDF, listen while following the score, close the score and sing it back, then re-open the score to check. Then choose another work, listen WITHOUT opening the score, then open the score and sing it back. Then finally choose another work, play the midi, sing it back and then open the score to check.
In both of these exercises I've said choose one work, but of course you could do more than one work at each stage until you feel confident with the process. And do all four stages each time you practise one of these exercises, until you feel confident enough not to need the initial stage.
Best of luck with the exam, Gwu. I'm sure you'll be fine. And if in doubt, make something up rather than not responding, at least you'll get credit for the attempt. But I'm sure you'll actually do a lot better than that.
Cheers
Katyjay