If you need to debug RNGBridge or want to see what's going on under the hood, first you will need a Serial to USB converter.
Hook its RX/receiver pin up to pin D4 of the ESP8266 D1 mini and open a serial terminal of your choice.
Make sure you set the baudrate to 115200 and start the terminal.
Now you should be able to see every debug output in your terminal.
The specific serial options are
115200 8N1, which stands for a baudrate of115200, 8 data bits, no parity and 1 stop bit.