I had one at 17 weeks. I had a few issues with mine but that's because there was supposed to be 2 doctors doing it, and it ended up being one doctor and one MW who didn't really know what she was doing. Saying that, all turned out ok. I went because of high risk from bloods so I got a full scan first then they done the amnio.
The doctor will put the needle into your belly, while looking at the screen to check where the baby is. Then they will put the syringe inside the needle part and pull out until they get enough amniotic fluid to test. That's it - it doesn't take long and it doesn't really hurt. You can feel it, but you're also watching the baby at the same time and concentrating on being very, very still. It can take them a little while to put the needle in because they obviously need to be very careful for it not to touch the baby, but remember that these people have probably carried out loads of these tests so are very experienced in what they are doing. The doctor that scanned me and that lead the amnio was really nice and explained everything in great detail and ensured it was really what I wanted. I also got the results back the next day so didn't have long to wait.
Hope all goes well, and please ask if you have any questions xoxox