Hey mate, first things first - Congrats on the job (hopefully) lol. I've passed 8 drug tests now without ever using my own urine. The only "drug" I do is weed so it really pisses me off that a coke head can just take a break for 2 or 3 days and they're golden but as I'm a daily smoker I'd have to stop for 2months to properly purge everything from my system and that's just not feesable.
When your asked to take the test it's in a private room, nobody will be watching over your shoulder and when you're finished you just place the cup in the little window/cupboard on the wall.
First step, source some clean urine, a trustworthy friend or sibling is normally the way to go for me. If you get it the night before, stick it in the fridge.
Step two, put water in a pot and heat it up, when the water is warm (not boiling!) Place the bottle of urine into the pot for a minute to warm it up effectively, just like you would with a baby bottle. (Do not microwave your urine!).
Step three, transfer the urine into a small portable bottle, I find the little plastic travel bottles you use to bring fluids like perfume abroad are perfect but any small concealable bottle will do fine.
Step four, tie or tape the bottle to your inner thigh or between your nuts and your inner leg (sorry for the vulgarity lol). This is vital as your core temperature keeps the urine at it's natural heat when it leaves your body, if the urine is cold it won't trigger the test and come back as a false positive. This is how the majority of people fail the test as the examiners know what cold urine feels like, they are fucking doctors lol.
Step five, lock the door, remove the bottle that is still warm and pour it into the cup. Make sure you bring the empty bottle away with you - do NOT leave it in the little bin in the toilet, they check those too.
It may seem a little impractical and it is slightly uncomfortable but I've done this exact method 8 times and have never failed a drug test and have gotten the 5 jobs I had to take one for (the other three times were for drug tests scheduled within the company but you're told when they are coming up).
I hope this helps mate and best of luck going forward!