At Heartlands they are done under Xray guidance.
You first of all swallow a mouthful of barium so they can see what sort of restriction there is currently.
Then they will add some saline, then another swallow to see what difference there is.
Once they are happy that the optimum is in there, you have to sit and drink 2 beakers of water to check there is no blockage, then you can go home.
Doesn't hurt a bit.