What You Need:
• A shallow plastic container with a tight-fitting lid, such as a food-storage container or lunchbox. It doesn't matter what shape it is, but look for one that's quite shallow rather than deep.
• A piece of watercolour paper. It doesn't matter what kind, though a thicker sheet works best as it holds more water.
• Greaseproof paper (baking parchment). Buy a roll, because you'll replace this every time you clean your palette.
• A pencil.
• Scissors.

What You Do:
• Cut a piece of watercolour paper to fit in the bottom of the plastic container. The easiest way to get a good fit is to place the plastic container on the sheet of watercolour paper, trace around it with the pencil, then cut just inside the line.
• Cut a piece of greaseproof paper the same size.
• Soak the piece of watercolour paper in clean water, then put it in the bottom of the plastic container.
• Moisten the sheet of greaseproof paper and put it on top of the sheet of watercolour paper.
• Squeeze out your acrylic paints onto the greaseproof paper and start painting.
• When you've finished a painting session, simply put on the lid of the container to keep the acrylics moist until you next pick up your brushes.

Tips:
• If you haven't got a suitable plastic container, you can use a baking tray or deep plate and plastic film instead. The disadvantage of using plastic film rather than a stiff plastic lid is that it's likely to touch the paint and so be more messy to take off and replace.
• If the sheet of watercolour paper begins to dry out, lift one corner, pour in a teaspoon or two of water underneath, then gently tip the palette so the water runs under the paper.
• If you cut the piece of greaseproof paper slightly smaller than the piece of watercolour paper, you needn't lift the sheets to remoisten the watercolour paper.

How Do I Use the Palette?
See: How to Use a Moisture-Retaining Palette for Acrylic Painting.