Youtube is a weird place to look for a picture
Not verified. Take care, do not short it. Best to only have one probe on the power connector, pick up ground from somewhere else.
Best way might be to clip the negative probe to something that is grounded, (make sure it does not slip), then poke the positive probe in the back of the connector.
Make sure your probes are in the correct sockets on the multimeter for measuring Voltage, if you plug them in wrong, you are likely to kill the PSU.
If you haven't done this before, it might be worth asking a friend with practice doing this to show you how to do it?