First you need to go to a supermarket and buy some Certo Gel. You usually ought to use both packets. About an hour and a half before youre UA piss everything out. Even if you can't piss anymore keep trying. Then mix a packet with a glass of orange juice. Drink it and take a piss. Right before you leave drink another packet and you should pass no matter what.
Just remember to piss only a little, mid-stream, in the cup. Most toxins are expelled in the beginning and end of urination.