[i]Simply connect the old Mac's Time Machine External Hard Disk to the new Mac using a USB cable before you turn on the new Mac. When you start the new Mac for the first time it will startup Setup Assistant (SA), SA will get to a point where it asks if you are migrating from another computer, select Yes and follow the on-screen prompts.
Don't overthink it. Yes, turn on the new iMac, begin the process and when prompted, plug-in (if not already hooked up) the TM backup and magic happens. The new iMac should "look" like the old one, once the transfer is completed. In my experiences it is not a real fast process, but it does the job.
