Read the data to watermark with in the usual way. There is no special procedure to read data just because it will later be used to watermark.
Then convert the data to a stream of bits or bytes in the usual way. There is no special procedure to convert data to bits or bytes just because the data came from an image or audio.
Once you have the bits or bytes, watermark with them in the usual way. There is no special procedure to watermark with bits or bytes just because the bits or bytes were derived from an image or audio.
