First, open a console to use airodump-ng to start sniffing for traffic. aireplay-ng and airodump-ng will run simultaneously, but in different consoles.
To start airodump, use the following command:
airodump-ng <wireless interface name)
Press Enter, It may take a few moments, but wait for the target network to show up in your list; an example is shown in the following screenshot:
Once your target shows up you can start aireplay. In a second console, enter the following:
aireplay-ng --arpreplay -b <target BSSID> -h <MAC address of connected client> <interface name>
The -b specifies ...