I just discovered this technique and effect by accident when I entered a "crappiest picture wins" competition. :p

Use the spray can to colour in the bit that you want the chalked effect to be. You can use different colours to mix and blend a bit. Make sure it's really grainy and uneven like stereotyped n00b spraycan fill - it won't work if you try to fill in evenly or solid. It also helps to use the largest spraycan, and also to spray a little bit of black and white over with the smallest can when you're done colouring. When you've finished, resize the document by 50% to 75% (anything inbetween works) and there ya go, it looks a tiny bit like it was chalked.