For the contents of the file, it needs to look like this:

[connection]
id=My Network With Spaces
uuid=
type=wifi

[wifi]
cloned-mac-address=
mode=infrastructure
ssid=My Network With Spaces

[wifi-security]
key-mgmt=wpa-psk
psk=My Network Password

[ipv4]
method=auto

[ipv6]
addr-gen-mode=stable-privacy
method=auto

[proxy]

Be sure to change id and ssid to be the name of your network, you don’t need to add quotes if there are spaces in your networks name. Also be sure to change psk to be your network password. When you are done use Ctrl+X to save the changes.

Now we need to restrict the file’s access rights because NetworkManager requires it to be only accessible by root:
sudo chmod -R 600 /etc/NetworkManager/system-connections/MySSID.nmconnection
sudo chown -R root:root /etc/NetworkManager/system-connections/MySSID.nmconnection

Now you should be able to automatically connect to your WiFi network during the next boot.