Take the film out of the camera, mark the number of exposures on it.
Read through the quick guide for the flash
Mount the flash and work with it some without film in the camera. In TTL mode, it will use the camera's exposure system, as stated, you don't need to worry about ambiant light. Just stay within the flash's effective range, as a guess it's probably good for up to 15 feet or so.
Load the camera either with the Kodachrome or color neg and go for it.