Put in the PSU, switch it off, and plug it into the wall.
DO NOT DO THIS UNLESS THE PSU HAS A DEDICATED ON/OFF SWITCH!
Without a dedicated on/off switch the psu will become live and you will have voltage (5v) being created at all times.
Its best to just keep bare skin in contact with bare metal of the case in this situation. Try to only touch the edges of the boards and only when you have to.