Once all the LEDs are soldered, Glue the soldered PCB (with the LEDs facing away from the loupe) to the lens side of the loupe using instant glue. Take precaution to alighn the PCB carefully. Now, connect the battery to the PCB copper bands. Observe the polarity while soldering the battery to the PCB.
I did not use a switch to turn the LEDs off and honestly it was an oversight. You could place an On/Off switch in series to turn the LEDs off, I just remove a battery from the battery holder to turn it off.