Black and white photography can be difficult to process and that too in fashion photography. With practice you can get the hang of BW but your picture should have the right lighting, the subject and the dress.
To maximize control when converting from color to black and white, it’s better to use RAW. Because the RAW file has a higher flexibility and better tolerance. Do not use black and white mode in the camera as it will not help you getting the right tones as it does not have much options. When using the RAW mode with BW together in the camera, the display on the LCD of the camera is in black and white, but actually has no effect on RAW files. Hence you could shoot in RAW+Jpeg.
You should know that conversion to black and white will bring a lot of noise and artifacts, hence better to shoot with low ISO for maximum quality.