I feel like many of the posts above offer piecemeal advice. I've flown to Bahamas and beyond from Florida. Here's a step-by-step.
1. Pick your destination in the Bahamas. Your initial landing needs to be a Bahamas airport of entry (AOE). There are lots of these, but not every airport is an AOE. Nassau, Bimini, Freeport, Exuma, several options on Andros, etc. are AOEs. Advise if you need to know if a specific airport is an AOE. If your destination is not an AOE, pick an AOE as an intermediate stop, then go to (2).
2. Before departing the US, you need to file an eAPIS. It isn't hard. I've done it a ton of times, but you have to navigate the website. You can leave from any airport, or even your backyard, in the US. You just need to file the passenger information on the eAPIS website. In the departure place, you need to pick the closest customs point -- this may or may not be the place you're actually departing from. If you're departing from somewhere else, just describe that place in the box. For example, departing from "my backyard" near FXE. Enter KFXE as the departure point and then note "my backyard" in the description box.
You should get an email reply indicating you are clear for departure. If for some reason, you or your passengers are suspicious characters, you could get an email asking you to report to the FXE Customs officers. It's never happened to me, nor have I heard it happening. But in theory it could.
You can also file your return eAPIS at this point. See below for instructions, but if you're worried about internet connectivity from the Bahamas, you can file now.
3. File a flight plan. I suggest you call FSS directly rather than filing online. If you file DVFR as your question suggests, then you just tell them so. If they ask you to estimate the time of ADIZ penetration, just guess to the nearest 5 minutes. They aren't going to hold you to the second. Once you take off, call flight service and activate your flight plan. While talking to them, ask for a squawk code. Use this when in the ADIZ unless you're receiving flight following and they give you a different squawk.
Also, tell them to put ADCUS in the remarks section of the flight plan. This may have no effect, but it can't hurt.
4. When you land in the Bahamas, exact procedures depend on the airport of entry. If you land at a bigger airport, you might have an FBO to go to and they might handle some of the forms and formalities for you. Assuming not:
Make sure you enter using a C7A "cruising permit" and not a C7, which is for commercial traffic. You'll pay a $50 processing fee for the C7A and then be able to fly to other airports in the Bahamas -- you might need to show your stamped C7A to customs officials at the other airports to prove you're a "domestic" flight, but otherwise you are good.
You might need several copies of the C7A. So either Google ahead of time and print out several copies, or the FBO/handler you use will help with this for you. You/your passengers might also have to fill out immigration cards.
5. You're in the Bahamas! Keep in mind that there's no night VFR, and that VFR requires that you remain in sight of the surface. Also, there are only instrument approaches to Nassau and Freeport. And if you want to leave the Bahamas, it needs to be through an AOE -- unlike the US, you can't leave from your backyard.
6. Assuming you're going back to the US. You need to file another eAPIS or rely on a previously filed eAPIS. Even if you've filed, you also need to call the port of entry at least an hour before departure to notify them of your arrival. At this point, you can also update your ETA (as long as the day is the same). If you've lost a passenger, you can supposedly advise this by phone, but if you've added any passengers, you need to file again since they don't have their info. Get the initials of the officer you talk to -- they'll normally give these to you as a matter of course, but ask if they don't.
7. Clear Bahamas immigration and customs outbound. You'll pay a departure tax of $29 per person outbound and surrender your C7A cruising permit.
8. File a DVFR flight plan outbound. Depending on where you leave from in the Bahamas, this could be challenging. If you leave from Nassau, Freeport, Exuma, or another place with a true FBO, this is fairly easy. Otherwise, you have to call Nassau or Miami FSS by cell phone or using the "special" phones in the airport terminals. Or, you can try to file in the air over the FSS frequencies on the charts.
9. On departure, activate your flight plane and get a squawk code from FSS. If you call for flight following and get another squawk, that takes precedence, otherwise use the one you get from the FSS for the ADIZ.
10. Land at an airport in south Florida. The farthest north you can clear customs from southern points (without special authorization) is at KFPR. In any case, land at the airport you called ahead to and ask to taxi to US Customs. They are usually listening on the ground/tower frequency for you. Unlike elsewhere in the US, you usually exit the plane with your passengers and bags and go into the Customs facility without further signal. I think the exception to this is Key West.
11. Fill out your normal immigration cards for US Customs and get cleared. Once you're good to go, you're back in the US and can fly around like a normal domestic flight.
