The simplest instructions can be found over at Thrifty Fun. There are just 3 ingredients - Plaster of Paris, powdered tempura paint, and water, and it takes about 24 hours for your chalk sticks to cure, so if you want to chalkpaint the front sidewalks on the weekend, you might want to make your chalks ahead of time.
The last time we made sidewalk chalk we used old egg cartons for the moulds, and the shape worked very well for toddler hands. A few more variations/suggestions, below.
• Marble Chalk. Make 'marble' chalk, by taking two contrasting colors, pouring equal amounts of each color into small paper cups (Dixie, or Dixie-like cups work well), and then with a toothpick, swirling the two colors together for a marbled look.
• Bi-color Chalk. Put a piece of waxed-paper wrapped cardboard down the centre of a Dixie cup, pour two different colors in on each side, let it set for a few hours and then slide the divider out before the chalk has hardened completely.
• Use glow-in-the-dark tempura paint, and watch what happens after the sun sets!
Making chalk is not a tidy process, but it is a fun one, and a great way to teach children about color, and about the riotous choices one can make with color when unfettered by constraints of permanence. You might not paint a room in the colors a child would pick out, but you sure can temporarily paint the sidewalk in them!