I see you are in the UK and since I've never been there I don't know the specifics on how it works, but I would imagine it's fairly similar.
You will go to the ticket counter/check-in kiosk and sign in for your flight, get your boarding passes (if you didn't already print them off at home) then you will take your checked bags to the designated TSA dropoff area where they will screen them and pass them off to the airlines to be loaded on the plane. You will not have to wait for them to be screened, once you give them to the TSO you're good to go. Then you would proceed to the TSA screening area. And here are my tips (DH works for TSA):
-Have your boarding pass and ID out
-Take your shoes off while in line (I bring socks if I'm wearing sandals) and the new policy is that you are going to put your shoes sole up on the belt, not in a bucket
-Make sure all non-medicinal liquids follow the 3-1-1 rule, take the plastic bag out of your carry on and put it in the bin, declare all medicinal liquids
-No metal on you including some belt buckles
-Unless you have one of the TSA approved cases, your laptop has to be outside it's case
And last but not least, make sure to give yourself plenty of time, you may not have an issue, but someone else may cause a probem that will slow everyone else down going through security.
If you have any questions about security please feel free to PM me or go to the TSA website at
www.tsa.gov.
Enjoy your vacation.