Hi,
Enable Messages on the iPhone.
Enable Messages on the Mac.
At this point you should be able to iMessages yourself (manually type the iPhone Number in the "To" spot on the Mac rather that rely on the Contacts App My Card).
Check the Messages App setting on the iPhone has "Send as SMS" enabled.
Add the Apple ID to the iPhone > Settings > Messages > Send and Receive section.
Accept the pop up on the Mac (and other devices)
The Missing instruction is here
On the iPhone go to the WiFi setting and make sure the iPhone is joining the same LAN the Mac is on.
Now set the Text Forwarding Option.
This should generate the Code on the Mac that you enter back on the iPhone.
NOTES
You cannot do the Text/SMS forwarding without setting up both versions of the App to sync iMessages.
Nor will it work unless the Mac and iPhone are on the same LAN.
This last bit is a bit like Internet Sharing on the Mac where the Mac's connection to the Internet might be Ethernet meaning you can Share that connection over WiFi.
In this case you are Sharing the iPhone's Carrier SMS connection over Wifi from the Phone to the LAN.
There can be issues if your Wifi part of the network uses it's own Router (and IP addresses) compared to those connected by Ethernet. It is a LAN with Subnets and you need to be in the subnet if that happens.

9:03 pm Saturday; November 24, 2018
iMac 2.5Ghz i5 2011 (Sierra)
G4/1GhzDual MDD (Leopard 10.5.8)
MacBookPro 2Gb (Snow Leopard 10.6.8)
MacBookPro 15" 2016 (High Sierra 10.13.x)
Mac OS X (10.6.8),
iPhone 6 iOS 11.x and an iPad (2)