The principle
Some tests measure what the kidneys clear in a whole day. So all the urine passed over 24 hours must be collected, in the container given by the centre.
The method
On the first morning, pass urine in the toilet and note the time: that is the start. From then on, collect all urine in the container, day and night, until the same time the next morning, when you collect the last.
Keeping and returning it
Keep the container cool during the collection and bring it to the centre the same day. Say if you missed any: an incomplete collection skews the result, and it is better to start again.