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You have several options when choosing a modelling clay to sculpt your face. Each clay is different, so choose the clay that best suits your purposes. Ceramic clays are water-based and easy to work with right out of the bag. It needs to stay moist while being worked, as it can dry out and crack. Ceramic clay can also be fire hardened to make a permanent sculpture. Plasteline clay is an oil-based clay that doesn't dry out, and can't be fire hardened. It is popular with special effects crews for its ability to hold high levels of detail. Polymer clays require an armature, or a wire skeleton, for support. They are weaker than other clays, but are good for painting. Polymer clays can be fire hardened, though they are not as hard as ceramic clays. Aside from the clay, you'll need a few other things before you can start sculpting. A good, clean working area is necessary, as are a few tools to help you add detail to your sculpture. You can buy sculpting tools from most craft stores. You don't have to buy specialized tools for sculpting. It may be possible to find other utensils that serve the same purpose. The main functions of your tools are to cut, scrape, and shape the clay. You can use sewing needles to draw fine lines in your clay, and add detail. If you know the person whose face you are sculpting, take pictures of them from all angles. Try to get some good straight shots of your subject looking straight at the camera. Take some from the side as well to get a good profile. If you're basing your sculpture on a famous person, find photos on the internet and print them out. Make sure you have a variety of angles to get a good idea of the person's proportions. It may help to draw some grid lines on some of your photos to help you see the relationship between facial features. Think about how you want the person to look in your sculpture. Think about who they are, what they do, and why you want to sculpt them. Use these questions to add emotion to your sculpture. Do a rough drawing of different expressions to get some ideas for how you want your sculpture to look. The drawing doesn't have to be perfect. It's just a tool to help you plot out your sculpture.
Choose your clay. Gather supplies. Study your subject. Sketch your design.