Below is the schematic that shows how the IC is wired to the ATMEGA328 microcontroller and the 4 digit 7-segment display with comon cathode.
Besides the MAX7219 you need only three other external components: two capacitors and one resistor.
The capacitors are here to reduce noise signals and cannot be ommited.
The resistor is used to set the limit for the LED's current. It should be at least 10kohm. The MAX7219 has to be powered with 5VDC. It is possible to use the +5V of the ATMEGA328 boerd. Both the ground pins of the MAX7219 have to be connected to the ATMEGA328 board.
The three signal lines (DIn,CLK,Load(/CS)) have to be connected to three digital outputs on the ATMEGA328 board.