they play the tonic chord and starting note. you then have 2 minutes to see it, and you can make as much noise as you like. then they play the tonic chord again and the starting note and you are off.
just don't freak out about it. thats the main thing
1) they'll always give you a sight singing in the key that you can sing, ie they won't give guys something too high.
2) it's all relative. don't be fooled by people who go, i can't sight sing because i play a transposing instrument. thats just a poor excuse for crap relative pitch. unless y'all have perfect pitch, theres no reason why sight singing shouldn't be easy. think of the melodic lines in steps and leaps rather than in actual notes.
thats about it really. oh and it's marked out of 10 and i got 10/10 for my last sight singing exercise
not that i'm bragging or anything. lol.