Here are the details of soldering and assembling a FlexiBall. 

There are four WS2812B-2020 LEDs on the front side. Make sure the orientation of each LED is correct. FPC connector should be on the another side of the PCB.

Tip: Flexible PCB is different from rigid FR4. The usual third-hand is not recommended because its teeth of clips makes dents on flexible PCB. Use one PCB holder or wise without too much force so that the flexible PCB is just held there without moving or bending.

After the LEDs are soldered, it is natural to connect two PCBs together.

Each PCB has three connection port. There are one input with a label of "I" and two outputs with a label of "O". There is a small gap on each connection port. Put two PCB as the above before soldering.

There are two cases of soldering. One is just to solder two connection ports on the FPC side only like the below. These are only ground on the FPC side. 

Another case is to solder solder the connection port on both LED side and FPC side. It is to connect both power and signal (on LED side) and ground (on FPC side). Below is one example.