An example of the technique described below.
1. First wash and iron white cotton an inch or two bigger than your intended finished piece. It should be 100% cotton, washed to remove any sizing (coating) and then ironed to be wrinkle free.
2. Lay your fabric on a paint proof surface - I use a piece of metal or acrylic but you could use the glass from an old picture frame (put tape around the edges so they aren't sharp) or in a pinch work on a chopping board you've covered with a wide sheet of aluminium foil. Spray it with water until thoroughly damp and smooth down.
3. Decide where your horizon line will be. I like to use the Jacquard Lumiere acrylic paints as they have two tones in the one pot - where you paint thickly appears quite metallic and gold and where the paint travels and soaks into the wet fabric is quite matte and the alternate colour. For a sky over the sea I start with halo violet-gold straight from the tub and paint a line along the horizon allowing the colour to bleed out into the wet paint.
4. Add colours to the sky. I've added halo pink-gold, burnt orange and sunset gold. However using pink-gold by itself can create similar effects if you use it both watered down and straight. Allow the wet fabric to merge the colours and keep your paint lines mostly horizontal - the more angular you do them the windier it will look. Spray more water as needed.
5. For the sea bunch up that part of the fabric and paint over the top folds. I've used halo blue-gold.
6. The colour will soak down a little way into the fabric. After painting the visible folds with both straight and watered down paint (to get different effects) re bunch the fabric to make the white ares stand up. Using a different colour paint the now exposed white areas (i've used pearl turquoise). Lastly re-bunch the fabric again to show any remaining white areas and paint those (i've used the very metallic 'super sparkle' for these. Keep bunching and adding paint / spray with water as needed until you have covered the fabric and are pleased with the result.
7. Lay your fabric out flat and admire your work. Ta Da!