Your veterinarian might ask you to take a urine sample from your cat because she shows signs of suffering from a disease such as cystitis or diabetes. A urine test will help diagnose these diseases. If you have one cat then the simplest way to collect a urine specimen is to remove most of the litter substrate from her litter tray and wait for her to urinate in it. Once she has urinated in the tray you can use an eye dropper to collect it. Important: the issue with this method is that you have to make sure that the tray is thoroughly cleaned before taking the sample in order to avoid contaminating the specimen. The tray will have to be washed, rinsed and air dried thoroughly beforehand.
You refrigerate the sample until you take it to your veterinarian. If you have more than one cat you will probably have to take the ill cat to your veterinarian to take the specimen at the clinic unless for instance they have separate litter trays for each cat, as advised by Jackson Galaxy, and you can guarantee that they stick to their trays (or confirm it through observation). You could use a motion activited camera trap pointed at the litter trays to check. These cameras are cheap and excellent on Amazon.
SOME MORE ON URINE AND URINARY TRACT:
Please search using the search box at the top of the site. You are bound to find what you are looking for.