When I've had to do this in the past, I was fortunate that my vet gave me a tad more guidance than it sounds like you've gotten.
Put that litter in an empty and clean box, and leave it and the cat in a closed room, ideally a bathroom. Check every few hours to see if the cats peed.
When I've had to do this I was able to leave the cat in the bathroom overnight; when things were more time critical in the vet gave me the option to just have them take a urine sample directly from the bladder in the office.
When I've had to do this in the past, I was fortunate that my vet gave me a tad more guidance than it sounds like you've gotten.
Put that litter in an empty and clean box, and leave it and the cat in a closed room, ideally a bathroom. Check every few hours to see if the cats peed.
When I've had to do this I was able to leave the cat in the bathroom overnight; when things were more time critical in the vet gave me the option to just have them take a urine sample directly from the bladder in the office.