Yupo is a synthetic material that can be used as paper. It is not paper, but a plastic that is made into sheets. You can order printed materials on Yupo to create water-resistant maps, menus, and other items. You can also paint on it. Because it is plastic, you can do things with a watercolour painting on Yupo that you can't do with regular papers. Erase watercolour paint, run colours together by simply tilting the paper, or add texture with plastic, salt or cloth.
Sketch your design on the Yupo paper with a pencil. The pencil will not show through the paint unless you press very hard, so it is best just to give yourself a rough guideline rather than a detailed image.
Mix your first colours. You can use watercolour paints and acrylics straight out of the tube of add water for a lighter, more transparent colour. If you add water, mix a little paint and water on your palette. You can also mix the colours together on the palette. Start with a couple colours, and add water to make thin paint.
Apply the paint to the paper and create a mixed look by tilting the paper. One thing you can do with Yupo that you cannot with regular paper is run two colours together, like drops of water blending. Add the watery colours to the paper next to each other. Tilt the paper to make them run together. Lay the paper flat when you achieve the blending you want. This creates a smoother transition between colours than brushing alone. You can do this with dark or light colours, as long as the paint is thin.
Add dark colour and texture to the painting with a palette knife. Squeeze a little bit of paint onto your palette. Use the palette knife to spread this thicker paint on the Yupo paper. This creates a thick, dark, slightly raised stroke of paint. Use the tip of the palette knife to dab paint into the Yupo, created slightly rounded shapes instead of strokes. This technique is used more often with oil or acrylic, but is possible with watercolours on Yupo. The thicker paint dries faster than watered paint, so you should squeeze only a little out of the tube at a time, to avoid wasting your paints.
Erase any mistakes so far. One benefit of Yupo is that you can erase areas of paint by wiping the area with a wet cloth or brush. You can dilute colours like this, or remove them completely, exposing the white Yupo underneath. Adding water this way can also re-wet the area so you can use more wet techniques after the paint has dried.
Add texture with salt, cloth or cling film. While the paint is wet, you can add texture with other items. Sprinkling a tiny bit if kosher salt on the painting creates small circles almost like stars. Pressing cloth down will imprint the weave of the fabric into the paint. Cling film laid flat will push the colours together to blend them, and make the paint take longer to dry, which lets you do more work while the paint is still wet.
Protect your completed Yupo painting by sealing it. If it is not sealed with fixative, you can continue working it, but any water could reduce your painting to a plain Yupo sheet. Sign the painting with a pen, or by scratching the paint away. Spray the whole thing with a coat of acrylic fixative or varnish. The acrylic-based products do not discolour with age and are best for protecting your work. Spray multiple light coats rather than one heavy coat; the drips from a heavy coat of spray fixative can ruin the painting you are trying to protect.
Yupo can also be used as a palette itself for thick paints, since it is plastic.