To create a stone tablet on which you can write hieroglyphs on, you can either use commercial air dry clay or make your own sand dough or salt dough. I love using sand dough because it dries into a somewhat gritty stone-like material.
All three materials can be colored by adding a bit of brown acrylic paint or poster paint. Knead the dough or clay until the color is evenly mixed.
Now you can try writing some hieroglyphs like the ancient Egyptians. Carve hieroglyphs onto the dough with a plastic knife, a small popsicle stick or the opposite end of a spoon. Use your little finger or the tip of a pencil to make dots and large depressions.
Below are resources to help you learn more about hieroglyphic writing and ancient Egypt: