You'll be told by a nurse first exactly what will happen then asked to sign a consent form.
Consultant should do a detailed scan of the foetus to check everything is OK.
Tummy swabbed with bright pink antiseptic.
Scanner thing covered in a sterile bag. Consultant uses this to guide the needle into a pocket of amniotic fluid away from the foetus.
Puncture of abdomen (this bit didn't hurt). Pause to make sure foetus doesn't nip back under the needle - if this happens, you have to wait, consultant may 'encourage' foetus to move out of the way and this is damn uncomfortable with the needle as well!
Into uterus (sharp cramp). Withdraw tiny bit of fluid and dump. Withdraw 20mls. Needle out, check foetal heart beat.
Rest a bit on the table - don't be frightened to ask for as much time as you need!
I was told the risk of miscarriage comes in 2 phases. 24-48 hours for mechanical trauma. Really important you rest up for the rest of the day - no lifting etc.
Next risk is from infection - about a week to 2 weeks later.
Hope all goes well!