• SIM800 5v/4v ↔ Arduino 5v
• SIM800 GND (either one) ↔ Arduino GND
• SIM800 SIM_TXD ↔ Arduino D8 (read through for the reason)
• SIM800 SIM_RXD ↔ Arduino D7 (read through for the reason)

Source: Quickstart SIM800 (SIM800L) with Arduino