There are as many different ways to paint a street painting, as there are painters to paint them. Styles can range all the way from tightly rendered photo-realistic work to very loose, Impressionistic styles of painting. Most directions share some specifics. One of these is that, in order to build up the pastel and try to get a feeling of something painted on the ground, rather than a rougher "chalk" feeling to the work, the artist needs to build up layers of pastels. It helps, even when doing the Impressionist style of paint stroke, to put at least a layer or two down underneath of blended pastel base. The base does a couple of things - it fills in the black of the asphalt (or whatever surface you are working on) so that the color is solid. Another thing it accomplishes is to give you a color to work on which helps blend in and make the next layer of pastel spread a bit farther and easier. When laying down that base of color, you might vary what colors you put down according to what you are painting. Bases of black and white are discouraged except in areas that will remain black or white - black pastel mixes in with any color and dulls it, and white makes any color a tint, thereby dulling it also. One thing to remember when working with pastel is that every color put down has an affect on the other colors it is mixed with. Unlike paint layers which dry and might have very little affect on future painting, pastel mixes up to the next layer, and can be mixed into the surrounding areas if care is not used. When laying down areas that are trying to indicate light and shadow sides, therefore, it is best to use colors that are those values for those areas i.e. use dark browns or something similar for the shadow areas of skin, and use peach and yellow tones for the lighter areas of skin.
As you build up subsequent layers, try blending less and less. On the top-most layers I blend the least or not at all. If I do any blending on these layers, it is with the actual chalk stick, lightly sweeping it over and over, color into color, and not blowing off the dust when I'm done. If the pastel marks are a tad too harsh, I'll lightly tamp it down with my fingertips to soften and settle the marks onto the asphalt. This section brings another question to mind: what do you use to blend with?
There are lots of different ways to blend and it is up to the inidividual artist what method they prefer or what finish they are trying to achieve. You can use latex gloves or fingertips - these have the added benefit of protecting your hands from the pigments, but they are not always comfortable for blending and can get very hot. Another tool is the chalkboard eraser. The eraser is great for filling in broad areas of color quickly. It is usually best to have an eraser dedicated to a color, such as one used only for black areas, as the chalk dust can build up inside the eraser and the color will affect what you do later. Some people buy cheap erasers and tear the felt strips out of them, using those for blending. I've seen people use just about anything else you can think of - pieces of terry cloth, pieces of charcoal chamois, make-up applicators, you name it. One of the better tools you have at your disposal comes for free in the pastel boxes - the strips of white styrofoam packing material they put in. Rip it into small squares and keep turning it at different angles to get the most use out of it. Rolling a piece into a thin long spike is good for detail areas.
My preferred blending tool though is my own hand. Hey, it makes for a filthy set of fingernails, but it sure gets the job done! Even when I use tools for different layers, such as the pastel packing strips, I like to finish up with my hands so I can smooth it down and make the chalk just right. One problem with using your hands is the fact that they get awfully dirty and you need them clean as you move from color to color, especially when working in lighter colors. Something handy that I like to use are the thick baby wipes. I always get the ones with Aloe Vera because they dry my hands out much less than the regular ones. They are a great relief when it is hot and dry and sore, and you can grab a wipe and moisten your hands. The thicker, better quality wipes, such as Baby Fresh, are well worth it - they go farther and have more moisture in them. A few of my friends use other things - one keeps a wet towel handy, and occasionally rinses it out. It's nice because you can always flop it over the back of your neck or your head when you are hot. Another sets up a small bowl of soapy water nearby and a couple towels. This was a great idea, just difficult in many places due to constrictions on size, and I sure wouldn't want to be the one who tips it over and soaks someone's painting - she usually sets it off to the side in some nearby bushes.
When painting such a large piece, you might wonder where to start - top? middle? bottom? I personally like to aim for something of importance, a focal point, right off the bat. It gives the public something to look at and gets me into the swing of things. Since I do mostly figurative work, there are usually faces in the top third of the painting, so this is what I work on. I'll focus on a few important things in the center section, then work above it and finish the top off, to the sides, and then down the painting. I usually finish an area completely before moving on. It's best to pay attention and not finish an area you are going to have to sit on top of later for more work somewhere else. While there are ways to move on top of the painting with little disturbance, the less risk, the better. Not to mention, if you are doing this out in public as a form of entertainment, covering up a finished portion is defeating the purpose.
To protect both the areas you are working on and your own self from the hot asphalt, you need something to sit on. Once again, personal preference is the key. Cardboard is cheap (free!) and causes little distrubance. Pretty much any smooth hard surface is the same - wood, plastic, etc. You can make a wooden platform that raises you a bit off the ground, though the added height that you have to bend down to work can bother your back. If you need to walk into the painting, wearing hard plastic or wooden soled shoes works well. Tennis shoes lift the pastel right up with each footprint. When I work I make several cards by laminating a color version of the painting I am doing. I might make a couple of these for different areas of the painting, and I might make them gridded first before I laminate, depending on how I am transferring the painting. These laminated cards have several purposes - not only do they stay clean (you can just wipe them with a baby wipe) and they are easier to hold than a sheet of paper, but they make great stepping stones for walking onto the painting. They are also handy for scooping up leftover pastels to move to another place.
Masking tape is handy when doing sharp, straight lines. You lay it down, paint on one side. Then lift it up and put down a new piece right on top of the pastel you painted, with the edge aligned and paint the other side. The masking tape only picks up minimal parts of what you've drawn and that can be touched up easily with your fingers or a tad more pastel.
True circles can be drawn by taking a piece of rope that is the radius of what you want to draw. tie one end of the string around a piece of pastel, tape the other end down securely into the center of where you want the circle to be and draw - it works like a giant compass.