I have this LED light strip which has AA Battery converter with a USB female output port. I will attach picture below. I would like to know how can I connect a SPST on/off switch to the light strip so that I can easily turn the lights on and off. Do I just tap into one of the power wires in the USB cord? Thanks for any advice.
Yes. You can splice in-between the battery pack and the female usb plug or the usb male plug and that black rectangle. Either the V+ or the Ground (unless both the shield and ground are tied together). Color is not guaranteed so make sure you use a multimeter to figure out which is which.
Personally I would add the switch to the case if you can.
No added fuse is needed. And for this you could probably use 24 or 22 awg. It may be thicker than whats already used.