3hr blood draw

Has anyone ever had to do this three hour blood draw with a special diet before hand? I was told my the nurse at my obgyns office I have to do this test and diet since my levels where too high when I did the normal draw for the Glucose test. I'm a little confused, do I do this diet just as it says or is this on top of how I normally eat? I'm pretty nervous about it and looking for any insite.