Dress design

There's really no single way to create a dress design, it comes down to how and what you wanna design.

One way could be to create a pattern for textile print: https://www.youtube.com/watch?v=lqm3_cQRrL0

If you want to create a print to add on a dress (like a print that can be used on t-shirts etc) this stream can be useful: https://www.youtube.com/watch?v=QsogjOu5KV4

You could also draw a dress using the pen tool, and depending on how you want the dress you can add different elements. 

Feel free to reply with more details so it's easier to suggest solutions 🙂