When iOS 8 was released and the feature wasn't there, we all waited for Yosemite. When Yosemite came out we realized we needed to wait for iOS 8.1. Now that you have those two installed (you must for this to work), there are just a few more steps to follow, but they are pretty simple:
- First, make sure you're signed into iMessage on your non-iPhone device. That could be your Yosemite-running Mac (on which you'd find this setting in the Messages.app > Preferences > Accounts); or on your iPad in Settings > Messages > iMessage.
- Jump over to your iPhone and confirm you're signed into iMessage and iCloud with the same account here, too.
- Still on your iPhone, go to Settings > Messages > Text Message Forwarding.
Enable “Text Message Forwarding” and your iPhone will relay SMS to/from your Mac or iPad - Flip the switch to enable it on one device at a time. Then look at that device – you'll see a confirmation number that you'll have to enter on your iPhone. This ensures you've got physical access to both devices at the time you're setting it up.
Confirm the link by typing the code and you're finished!
Once you've done this, you're good to go. Happy texting (and maybe try to encourage your Android-using friends to stop using SMS and just get an iPhone already. You know they want to!).