Choosing a base heli and camera
When choosing a base helicopter to modify, keep in mind the power of the engine relative to the size of the craft. Stability and agility are also major factors. The most stable Air Hogs helicopter, the Twin Thunder, is not agile and the frame does not easily support additional weight. The Reflex Micro Helicopter is a better choice due to a powerful engine and ease of modification due to the placement of the landing skids. The Reflex is a highly maneuverable craft that can be landed safely even with a miniature camera added. Your camera should be lightweight and fit easily between the skids. A cellphone camera that weights less than six ounces easily fits the bill, as do more expensive, higher quality options such as the Panasonic SDR-S7.
Modification of the craft
You will want to attach the camera securely to the landing skids with tape or adhesives. Align the iris of your camera in the direction you would like to record. This can be behind, to the front, or below your craft. If using a Reflex, keep the bulk of the camera towards the front to facilitate forward movement. If you desire a permanent modification, you can glue the camera directly to the underside of the fuselage or to a swivel to facilitate multiple camera angles. Once you have aligned your camera between the skids, you may notice some of the bulk reaches below the frame. This is acceptable, but remember the overhang when you land your craft.
Learning to fly again
You will undoubtedly discover that your helicopter flies differently with a major difference in weight below the fuselage. You will need to practice with this new design prior to any serious flying as it is possible you could damage your helicopter or camera as you learn how to fly the combined machines. Take care to not fly over people or living things until you are certain the camera is securely attached and you are in full control of the craft. Activate your camera prior to takeoff, and set the auto-timer for any still photos you plan to take.