I made a Joann's run and luckily the one near my house has a good selection so I was able to find watercolor paper and watercolor paints.
The watercolor paper was 9x12 which is a little big for my 3 year old (he tends to make smaller pictures) so I used my rotary cutter and ruler to cut the pages in half so they'd be 6x9.
Once I had some cut I laid them out so they'd overlap with only an inch at the top of each page showing. This isn't necessary but it was much quicker to be able to make a bunch at once instead of one at a time.
Next I added a dot of paint, starting with red, to the top (or bottom) of each page.
After I was done with red I did the same with all the other colors.
After I finished adding all the colors all that was left was to let the pages dry. Once dry they can be used with a damp paintbrush. Dorian loves playing with these now. Next time I make a batch I'll need to do it after I put him to bed since the entire time they were drying he was asking every 5 minuutes if they were ready yet.
Here he's using one of the full 9x12 sheet I originally made (he decided he'd rather swirl the colors together than make a picture).