Here is the activity. I say, "Write me the letter/letters that make the sound _____", and make a sound that they have learned. Students write the sound with the marker on the paper. You can see below the sounds that we were practicing. For ch, tch, and or, ore, the prompt was "write the two ways you know to spell the sound______ " (/ch/ and /or/). They LOVED it!
If you have never used Crayola Color Wonder things, you should. They often come with coloring books, but I found the packet of blank pages below at Walmart. Well I found it in the bottom of my color book drawer at home, but I am assuming it at one time came from Walmart. The markers appear to be clear, and will not write on furniture, clothing or paper other than the Color Wonder Paper. On the Color Wonder Paper the colors appear magically a couple seconds after you write with them. So Crayola Color Wonder Paper... a big hit with toddlers and first grade writers alike!