OK here goes
You will go into a room and lie on a table/bed - usually an Xray bed. You will have electrodes attached to monitor your heart - the same as an ECG if you have had one of those.
There will be a couple of straps put round you to keep you on the bed.
The bed is then tilted - the position is as if you were standing up, them lay back at an angle - sort of like lying on a VERY steep hill, so your head is still at the top and your feet at the bottom but your head will be further back than yoru feet.
Then the room is dimmed andd you just lie there. In most cases nothing happens which is a negative test. There will normally be a Dr and a cadaic physiologist in the room. It will be the most boring half hour of your life.
But sometimes things do happen, your heart rate might change, you may faint - these are positive tests.
The idea behind the test is that your sympathetic nervous system is turned down and control hadded to the parasypathetic is given temporary control.
If the test is positive the bed will be tilted back to the normal position very quickly. If you think you have fallen asleep ande wake upp with a start on your back you have had a positive test.
That's the basics, you may be given drugs to try to induce the response but not always.