I can help you out here since I work for the Post Office

This is the drill I went through...

go to the bathroom they asked you to empty all your pockets and such, then wash your hands. they shut off the water and the tester goes and stands outside the door while unlocked and then you piss into the cup.

The tester comes back in and test with the stick the degrees and if its the right yellow hue to accept. then procedes to put a small amount of urine in another cup that then test it guess its mixes in with some chemicals for each of the drugs like weed coke opiates and such.

its pretty simple i passed with no issues at all drank this stuff called the Goldanidator and a redbull i was set man (the redbull was to get more yellowing in me)