You have it exactly right. If you're flying VFR, file a VFR flight plan for both the departure and return (no need to activate them...but confirm not needing to activate with the language of your TFR Notam). Just prior to departing, call ATC (the controlling authority...in our case around NYC, its the NY TRACON) on the radio if you can get them, or on the telephone and tell them who you are, where and when you're departing, and where you are going to (and your route if it is not directly out of the TFR). You will get your squawk code and Com frequency from them. Dial in the code on the ground as soon as you start your airplane. Take off as usual, and contact ATC on the frequency you were given as soon as you are safely up in the air.
The return is easier. In addition to having the flight plan on file, get flight following prior to approaching the TFR boundary. That way you will have a squawk code. ATC will handle you as they would normally for flight following except that when they terminate radar services (typically when you have the destination airport in sight), they should remind you NOT to squawk 1200, but, instead, keep your assigned squawk code to the ground. Actually keep it tuned until you turn off your transponder.
The only thing I can't help with is the case where you can't contact ATC directly by either radio or telephone. In that case I expect that you would call Leidos Flight Service and have them contact ATC to get the code and frequency prior to departure. The same as having FSS act as an intermediary with ATC in getting an IFR clearance. But I've never had to do that for a TFR, so I can't say for certain.
