For Standard version tank:  

  1. Unscrew to remove the top cap;
  2. Add your preferred e-liquid into the tank through the filling hole;
  3. Screw the top cap back.

For Child-proof version tank:

  1. Press and unscrew to remove the top cap;
  2. Add your preferred e-liquid into the tank through the filling hole;
  3. Press and screw the top cap back.

Note: The pressing and twisting operations in each step are performed simultaneously.