Although decorative weights are typically used to secure tablecloths when dining outside, they are pretty enough to add to your curtains to keep them still.
Decorative curtain weights come in both clip-on and magnetic. Whatever style you choose, you will need several for each curtain to keep them still on a moderately windy day.
You can secure the bottom of your curtain with rope or wire, which will ensure the fabric never blows around and makes it like a sliding privacy screen.
Use a length of rope or wire long enough to tie around each corner of your outdoor space. If you have an area with small corner posts, you can secure it around those.
Choose washers less than 1 inch in diameter, and sew them into the hem on the bottom of your curtain about 12 inches apart. This only works for sturdy fabrics like canvas.