I'm writing this as systemd seems to be getting stronger and is replacing initd little by little.  Now if like me you have one or more  Linux systems that do not run a desktop environment.  You may find yourself using a few apps that normally require a GUI to be configured.  NetworkManager is one of those, but does allow you to use the CLI (nmcli).

Here's how:

Assuming you already have everything installed :)

systemctl enable NetworkManager

Will start networkmanager at boot.

nmcli d

will list all interfaces controlled by networkmanager.

To add a connection eg wifi

nmcli c add ifname wlp5s0 type wifi ssid <your ssid>

This will create the file

/etc/NetworkManager/system-connections/wifi-wlp5s0

Which will contain the skeleton config (non working)

[connection]
id=<your ssid>
uuid=b083dd98-f1e0-4bc5-bff6-56c4a1b56e2f
interface-name=wlp5s0
type=wifi

[wifi]
ssid=1027251N
mode=infrastructure

We now need to configure the connection.  (additions highlighted)

[connection]
id=<your ssid>
uuid=b083dd98-f1e0-4bc5-bff6-56c4a1b56e2f
type=wifi

[wifi]
ssid=1027251N
mode=infrastructure
mac-address=00:23:14:B7:57:A0
security=wifi-security

[wifi-security]
key-mgmt=wpa-psk
auth-alg=open
psk=<your ultra secure key>

executing

nmcli c reload

will/should now connect you :)