12 week scan?

I can't remember the name of the specific scan to save the life of me but it's the one where they measure the fluid to see if baby is high risk for downs and other genetic Conditions. Well I have mine tomorrow and im scared and don't know what to expect. I opted out for it when I had my daughter 2 years ago but I decided to just do it this time. Anyone have any advice for what to expect and how theirs went? Thanks