FYB,
Do the following:
In TWS enter "USD" in the "Underlying" column then select "Cash" then select "GBP" and it should appear as the below screenshot.
Then click on "Order" and select "Sell" when another window will pop up as shown in the second screenshot. I have highlighted in red the sections that need to be as shown with the exception of the quantity which will be your choice of how many USD you want to sell. Then just hit transmit and a few moments later it should be complete.
Paul