Glucose test

So i have to be up early in the morning to take the glucose tolerance test to see if i can go on metformin since i have pcos and endo and my weight isn't budging no matter what i do 😧 my dietician suggested it and my gyno thought it was a good idea so she's getting me to do the test before prescribing it but i have no idea what to expect from it or how they even do the test

(It's the pcos that's causing problems since the endo was removed when i had my laparoscapy 8 weeks ago)