This cannot be stated enough. We must start everything with prayer. Ask God to give you a good location, ask Him to prepare hearts and ask His blessing on the event.
Ask for permission to do the chalk painting on the street/parking lot. Assure the person you are asking that it does wash away with the rain. Once you have secured permission, it's time to plan.
For our event we did it on our church parking lot on a Saturday afternoon, we are in a storefront church next to a busy highway, so we went all out, giving away items (we did a seed packet we made ourselves that will be shown in an upcoming idea), food (Hot Dogs and Sodas), face painting for the kids, Balloons, a clown that entertained the kids and stood by the road waving people in and two artists doing chalk paintings, (My wife and I).
You may not be able to do all of that, you may just want to go to a busy store or sidewalk and get permission to do a painting and just give out a hand out for your church. It is best to have at least two people in on this project. It is very time consuming for the artist so it helps if you have another person to aid in speaking with the people.
Now you've got your location, your event is planned and it's time to prepare the piece. Start off by creating your design. I recommend what I call a "soft cell" design. This is a design that is not overtly Christian, but can have a significance that you can talk about.
For example, While I love the Lord with all my heart, I would probably not use a picture of Him. People might see it and say, "Oh, a picture of Jesus" and walk away quickly, figuring someone is about to beat them over the head with a Bible. I chose a picture of a frog, and my wife did butterflies and a sonflower. The reason for these choices is simple, The Sonflower (yes I spelled it wrong on purpose) is a symbol of our new church which is encouraging people to grow in the Son, this design tied in with the seed packets we were giving away. The butterfiles and the frogs start out their lives as something else and become New Creations. This can lead into the sharing of a testimony. I find these designs open doors.
Once you have chosen your design, draw it proportional to the size of your finished piece. My frog piece was 8'x8', my wife. who had never done this before tried smaller 4'x4' designs but did beautifully with both. When you have finished your drawing grid your design into 1"x1" squares, see below
You will need a chalk line, a tape measure, sets of colored chalk pastels (we got ours in sets of 48 colors for $5.00 at out local AC Moore store, seven sets covered us for three pieces with quite a few left over), a few sidewalk chalks for filling in large areas, a good set of kneepads, and several pairs of cheap garden gloves for protecting your hands and blending large color areas. I also highly recommend sun screen. Also prepare any publicity materials you may need.
Start off by marking off your square using the tape measure and chalk line. After you have the square set up, mark off your grid, (1'x1' squares). You are now ready to begin draw your piece. copy what is in each of the squares of your plan drawing. Once your sketch is complete start in the upper left corner (upper right if you're left handed) and begin to fill in the colors. I recommend laying down your color areas a little at a time and then blending them with your fingers. Since you will be working on asphalt, you really have to work the chalk in. If you are using gloves you will need several pairs since they hold the color and will smear other areas. If you are using your bare fingers, you may want to wrap masking tape around them (One of these pieces takes three to eight hours, depending on size. That can really chew up your finger tips.)
The following are a few of our pieces. Keep in mind that my wife had never done this before and she came away with excellent results. Try this yourself. It's rewarding and fun and it may just open a few doors to the Gospel.
