I used to be a sail maker, and we always taped everything together first with seam stick tape. It's a double sided tape with a backing strip on one side. It's available in different widths. My choice would be 1/4". It's very light weight btw.
Bainbridge Aquabatten and Challenge Sailcloth sell it. An account might be required to purchase from either of these two companies. I'm sure other sources have it.