Step 1: Gather your materials...
What you will need:
CONCRETE - http://www.quikrete.com/index.asp
QUIKRETE® Concrete Mix (No. 1101) is the original 4000 psi average compressive strength blend of Portland cement, sand, and gravel or stone. Just add water. Use for any general concrete work. (Verbatim as posted on the Quikrete site) Resist the urge to use heavy duty concrete, as it is very chunky. For those familiar with mixing their own concrete instead of using a mix, by all means do so. We use Quikrete because it is available in our area and has proven to be of quality for the items I've made.
Though many home improvement stores carry ready-to-mix concrete in 80 pound bags, it is also available in other sizes, depending on your preference as well as ability to lug it around. Be sure to allow store employees to help load the larger bags into your car. There are many types of ready-to-mix available, choose accordingly. I prefer Quikrete (mainly because it is readily available in our area) and Quikrete Vinyl Concrete Patcher, but these products are mere suggestions. Nothing is cast in stone. Yet.
COLORING - Not a necessary item at all, though coloring concrete is quite fun, and easy! Check out liquid cement colors near the concrete section of your local hardware store. A 10 oz. bottle will color quite a bit of concrete. If you want to maintain color consistency in your projects, consider making up large bottles of colored water for your project, and be sure to keep a lid on the container of mixed water. Shake well before using. Don't get carried away by dumping in more colorant than recommended, or your concrete will not set properly.
MOLDS - An endless supply of molds, containers and other ideas are available everywhere. Scour yard sales, thrift stores and other thrifty places for interesting shapes and sizes. Don't stick to bowls, use your imagination. You could even make your own. Try not to choose anything with great detail, as you may be disappointed. For finer detail, use Vinyl Patch mix, which has far less bumps and bits of rock, or use a good cement and sand recipe.
Just about any container can be utilized as a mold for concrete, provided you are able to get the finished product out of it. Bowls, cups, milk cartons, jugs, the ideas are bountiful. At present, I've found much delight in selecting unique glass containers from second hand stores and yard sales. If the finished item cannot be dropped or dumped out of the mold, after the concrete has fully set up, simply (and gently) tap the glass to crack or break it from your concrete creation, then rinse off the glass and be sure to take it to the recycling center.
Plastic, stainless steel and other materials release from the cured concrete easily when non-stick spray is applied to the mold prior to adding concrete.
NON-STICK COOKING SPRAY - Yes, release agents are sold specifically for the purpose of mold release when using concrete, but quite frankly, a cheap can of non-stick cooking spray works just fine. Use it generously to ensure your project will slide out of the mold. I've used both generic and Pam brand cooking sprays with success. Thanks to the many wonderful Instructable folks who have also suggested using other agents such as WD40 and perhaps even motor oil.
WATER - Necessary to mix with the concrete. Not too hot, not too cold, not too much, not too little. Perhaps my 'luck' has been the love of making mud pies as a child. Think Goldilocks, and mix well.
RUBBER GLOVES - Nothing fancy needed, but you should wear them. Be safe, not sorry. Concrete poisoning is no fun, and it's not pretty. I know this from personal experience.
EMERGENCY MOLDS - So you've mixed a pristine batch of concrete, you've sprayed the mold and you're in the process of filling it. Whoops, not enough concrete! Quick, dump it out and reach for another mold. Keep one close by for this very reason, and don't forget to spray it first. It is better to make a bit more than to end up a bit short.
A POKER - You'll need something about the circumference of a pencil to poke out air bubbles.
A LARGE SPOON - Or any similar item to mix the concrete. My favorite? A skinny garden trowel. Keep your eyes off items in the utensil drawer of the kitchen unless you no longer wish to use it on food.
BUCKETS, MEASURING CUPS, MISCELLANEOUS 'TOOLS OF THE TRADE' - Obtain inexpensive tools and reserve them for concrete projects alone, as they will become tarnished with concrete. Don't be wasteful. Clean and re-use your tools.