How do I send text messages (NOT iMessages) from my Mac?

I recently bought my first Mac, after having been impressed with my iPhone 7 Plus for the past year. On my iPhone I can send both iMessages and text messages by switching on the "Send as SMS" option in Settings.

On my Mac the Messages app works great, except for when I want to send a message to a non-Apple user (most of my friends are on Android). When I try do do this I get an error like the one below:

Your message could not be sent.
PHONE_NUMBER is not registered with iMessage.

I feel like I looked everywhere in System Preferences, but cannot find a "Send as SMS" option. Surely this is possible? How?

Yes, it is possible! However, you do need to set things up first.

Follow these steps:

1. Sign into iCloud on your Mac and iPhone using the same Apple ID
2. On your Mac open the Messages app
3. On your iPhone go to Settings > Messages > Text Messages Forwarding
4. You should now see your Mac listed - tap on the toggle to enable it.
5. You’ll see a prompt asking you to enter the code shown on your Mac.
6. Enter the code

Now when you add a non-iPhone mobile number into Messages on the Mac it will be highlighted in green - this is an indicator that you are sending a text message and not an iMessage.

NOTE: As all text messages sent from your Mac are now forwarded via your iPhone, they will count towards any associated costs for your mobile plan (if any).