The first time we did this, I found I didn't have enough chalk and it all got drawn right out of existence. Kids were running around looking for tiny nubs of chalk. I needed to show up with more next time.
Beforehand I would sneak out a bit early and put some white outlines on the ground - lettering if we were advertising something (in this case the 10-10-10 Global Workday for the Green Schoolyard team and the message "Together we can... Make Art!") or, a basic design in the case of the courtyard, where we had a round space and I wanted to divide it into sections and then let the kids loose.
The kids would then start coloring in the lines, or adding things, or go far afield and create their very own drawings. All of which were wonderful.
Some of them put their own characters out there for everyone to see.
I also created some drawings myself and then asked for help finishing them.
If one kid had to leave, another one would often pick up and keep going. This tree had a lot of people work on it.
Finally, it was necessary to color the drain since it was in the middle of the sun's face.
Things to think about when doing chalk drawing:
1. Make sure you have enough chalk. It disappears.
2. Backs of necks can get sunburned since everyone is looking down - I tried to use shade whenever possible.
3. One of our chalk days was also picture day, and I'm really hoping the kids all got the chalk off their noses before their pictures were taken.
4. A light coating of hairspray afterward will get the chalk to stick a bit longer.
5. Next time I'm going to wet the chalk so we can get some more vibrant colors.
And finally, once again, whenever you put out art supplies for kids, let them do their thing. Each child's Creative Beast and it will come out to play if you let it. As long as they are being appropriate and respecting one another's projects, let them go. Then, talk to them and acknowledge what they are doing. This is huge. Putting out the supplies gives permission, and your acknowledgement gives them encouragement. Just notice what they are drawing, or ask them questions. No need to draw anything "right" or to "finish" anything. This is a totally different mode of thinking.
Oh, one other thing - many kids will come out of this with a fair amount of chalk on their behinds from sitting on it.