To condition your clay start by cutting the block of clay into thin slices using a straight blade. Roll over each slice with a clay roller, fold it in half and repeat several times until it starts to become softer. Continue conditioning with your hands, smooshing, rolling, and folding the clay until it becomes warm and pliable. You can condition the clay completely by hand or finish conditioning it with a pasta machine. Be sure to always insert the clay through the pasta machine fold first to prevent air bubbles from becoming trapped between the layers.
Here's a short video showing how to condition polymer clay.Softer clays such as Sculpey III and Pardo can usually be conditioned by hand by simply kneading them for a few minutes. Using a pasta machine makes quick work of conditioning clay and you can usually find pasta machines for a reasonable price in most craft stores and online sites that sell polymer clay.
Firmer clays can be crumbly and difficult to condition. With these clays you need to start warming the clay and conditioning by hand before trying to run them through the pasta machine to complete the conditioning process. One way to soften the clay is to mix in small amounts of translucent clay. However, if you add too much you may change the color of the clay. You can also add a few drops of liquid clay and knead it in thoroughly. Only add a few drops at a time or you will end up with a goopy mess! Yet another way to warm your clay in preparation for conditioning is to place wrapped blocks of clay on a Ziploc bag filled with 1/2 cup of rice that's been microwaved for 30-45 seconds. The rice will stay warm for up to an hour. You may want to put a cloth between the bag of rice and your clay to prevent overheating. You don't want the clay to begin curing. One method I use to warm stiff clay is to sit on the clay or put it in my pocket (wrapped in it's original package or in a bag). Just never put polymer clay in the microwave for any reason.
The heat of your hands can make some clays softer and softer while you're working with them. If your clay becomes too soft and difficult to work with you may need to set it aside for a while and let it cool off. Some people even put it in the refrigerator for a short time.
You can make soft clay firmer by a process called leaching, which removes some of the plasticizers from the clay. While leaching will make the clay firmer to work with, it can also lessen the strength of the finished piece. To leach your clay sandwich thin pieces of polymer clay between several layers of printer paper. Place your "clay sandwich" on your work tile and set a heavy book on top. Check your clay every so often until the desired firmness is achieved. This process can take anywhere from a few hours to a few days. Replace the paper when it appears saturated. It will look like a grease stain on the paper. And remember not to leave unbaked clay on painted, varnished, or lacquered surfaces, as the clay will damage them.
When working with different colors of clay start with the lightest colors first whenever possible. Light colored clays can be difficult to keep clean since they will pick up anything on your hands, tools, or work surface. This post has more tips on working with light colored clay: Tips For Sculpting With White and Light Colored Polymer Clay. Baby wipes work well for cleaning your hands not only in between colors, but every so often while you're working with your light color. I also wipe down my tools to be sure there are no bits of darker clay on them. Just be sure your wipes are lint free and don't leave fine particles on your hands, as these will quickly transfer to the clay.
My preferred work surface is a ceramic tile. Tiles come in a variety of sizes and are fairly inexpensive at home improvement stores like Home Depot or Lowes. They're smooth and easy to clean and don't affect the clay. The tiles are nearly impervious to scratching and can go right from your work table to the oven. I have tiles in several sizes for different sized projects.
If you don't want your clay to stick to your tile parchment type deli paper works quite nicely. Placing your project on this allows you to move it around without distorting it. I find this very useful when cutting out leaves and flowers. I can cut them out on the paper and easily remove them to place them on my sculpture. you can find this deli paper, also known as patty paper, in bulk at grocery warehouses and restaurant supply stores. Baking on this paper also helps prevent shiny spots from forming on the bottom of your piece.
I hope these tips have helped you learn some of the basics of working with polymer clay!