1. Make sure your devices are compatible.
2. Turn Bluetooth on, on all your devices.
Apple released Yosemite (OS X 10.10), last week, and iOS 8.1 today. Together, with supported hardware, these two OSes enable the Continuity feature-set; one specific feature is Handoff. In a nutshell, these features let you start a task on one device, such as writing an e-mail or document, and pickup where you left off on another. They also allow your phone to handoff phone calls and SMS text messages to your Mac.
It took a little digging to figure out how to coordinate this. Here are the detailed details [sic]:
In a nutshell, your devices need to be on the same WiFi network and Bluetooth needs to be turned on for the them to see each other. FaceTime needs to be turned on and logged into the same iCloud account. It seemed that FaceTime didn't realize that email@example.com, firstname.lastname@example.org, and email@example.com were the same account. I had to uncheck the latter two before I could check the "iPhone Cellular Calls" preference in FaceTime on my Mac.
When my first non-iPhone SMS came through, I had to enter a PIN on my Mac that appeared on my iPhone to confirm I had physical access to both devices.
So far, Continuity seems to be working fairly well.