Cut the head off a 5v power supply (phone charger, Raspberry Pi supply etc) and look for the red and black (power and ground) wires if it’s a USB type cable (there should also be a green and a white one in there) or if it’s a twin-wire power-only cable, find the one with the painted stripe on it (this is the positive wire) and connect to the +ve terminal on your multimeter.
The black cable should be labelled COM (it *might* be labelled negative or ground, but here’s one example where this is incorrect labelling!). Connect this to the *positive* terminal on your LED. So the power is basically going from the adapter through the multimeter and into the +ve terminal on your LED light.
Now connect the -ve/ground wire from the adapter to the -ve terminal on the LED. Set to full brightness and switch on.
Depending on which setting you have on your multimeter, and depending on the current draw, you may need to move the dial to get a reading. For example, if your LED is drawing up to 20mA you’ll need it set to 20m (this is unlikely to be the correct setting – a single LED can pull 20mA easily). If it’s drawing up to 200mA (possible, but unlikely depending on how the LEDs are wired) you’ll need it set to 200m on the dial. Anything over 200mA (up to 10A) you’ll need the dial set to 10A for a reading to show up (this is where I’d expect the values to appear).
Don’t forget that at 200m, the reading will be in milliamps (so let’s say it reads 900 on the 200m setting – this is 900 milliamps – not 900 amps!). But depending on your multimeter, you might also get the reading 0.9 on the 10m setting.
Anyway, once you’ve done this, you’ll have actual real current readings (rather than over-estimated theoretical maximum ratings). Simply add up the current rating of each LED and this is the current rating you want for your power supply (it’s possible you might just be able to run all six off a single 3A raspberry pi type power adapter, if they each pull up to 500mA but I equally wouldn’t be surprised if all six LEDs can light at once, they draw an amp or more – it all depends on whether the LEDs are being multiplexed by those onboard microcontrollers).
