I remember my young sister getting into trouble for using her new set of felt tip pens on the kitchen wallpaper rather than in her colouring book. Many years later when working in a nursery I pinned large sheets of paper to the wall, and left jumbo markers around, hoping that the three year olds would do exactly the same!
We all know that young children who are learning to speak "play" with the sounds of the language that they hear around them and adults will encourage their early efforts. The babbling sound of mamamamama and dadada and the other not-so-recognisable sounds that they make have us smiling, cooing, and having baby conversations. Just as children learn to speak by practising with the sounds of language, so they learn to write by experimenting with print. Even if an adult doesn't make them explicitly aware of it, they see writing and print all around them, in the streets on road names and shop signs, on the television, in shops, and in books, newspapers and magazines. If they are lucky they will be read bedtime stories too. Even very young children absorb the fact that these strange marks are obviously important, and these marks are made up of lines, circles, squiggles, dots, and some spaces. Provide chalks, pencils, pens, crayons, and a surface on which to make marks, and watch the early stages of writing develop in front of your eyes. (And buy washable writing tools, just in case the wallpaper or lino becomes too attractive.)
