$ lsusb
Bus 001 Device 004: ID 7392:7811 Edimax Technology Co., Ltd EW-7811Un 802.11n Wireless Adapter [Realtek RTL8188CUS]
Install the driver module:
$ pacman -Sy hostapd-8192cu wpa_supplicant
$ ip a
3: wlan0: <NO-CARRIER,BROADCAST,MULTICAST,UP> mtu 1500 qdisc mq state DOWN group default qlen 1000
link/ether 00:aa:00:aa:00:aa brd ff:ff:ff:ff:ff:ff
# Assuming that the wireless device is wlan0:
$ cat <<EOF > wpa_supplicant-wlan0.conf
ctrl_interface=/var/run/wpa_supplicant GROUP=wheel
eapol_version=1
ap_scan=1
fast_reauth=1
EOF
$ wpa_passphrase MYSSID passphrase >> /etc/wpa_supplicant/wpa_supplicant-wlan0.conf