Arduino IDE for Windows/MAC (1.8.10)                     
FTDI Adapter FT232RL 3.3V
                   
Needed libraries: (recommended versions in brackets)                    
esp8266 by ESP8266 Community (2.4.2) <<-- THIS IS IMPORTANT!                    
Arduino_JSON by Arduino (0.1.0)                    
Adafruit NeoPixel by Adafruit (1.3.4)                   
Adafruit Unified Sensor by Adafruit (1.1.1)                   
ArduinoHttpClient by Arduino (0.4.0)                   
ArduinoOTA by Juraj Andressy (1.0.3)                   
DHT sensor library by Adafruit (1.3.8)                   
DS3232RTC by Jack Christensen (1.2.9)                   
IRremoteESP8266 by Sebastien Warin (2.7.1)                   
Time by Michael Margolis (1.6.0)
             
Also includes the Timezone library from Jack Christensen.

              
Compiler-Options: (recommended)        
Arduino IDE for Windows/MAC (1.8.10)                   
Board: "Generic ESP8266 Module"                   
Upload Spee: "115200"                   
CPU Frequency: "80 MHz"                    
Crystal Frequency: "26 MHz"                    
Flash Size: "4M (3M SPIFFS)"                   
Flash Frequency: "40MHz"                   
Reset Method: "ck"                   
Debug port: "Disabled"                   
Debug Level: "None"                   
IwIP Variant: "v2 Lower Memory"                   
VTables: "Flash"                  
Builtin LED: "2"                  
Erase Flash: "Only Sketch"                   
Port: "your USB port"

While the Arduino IDE is compiling the firmware, the ESP must be put into firmware upload mode.
To put the ESP chip into firmware upload mode, you have to press both buttons (RESET & FLASH) simultaneously and then release the reset button first.