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Pieces of material used in mosaic art are called tesserae.  They can be glass, stones, porcelain, seashells, or whatever else you can find.  You will also want to gather together any objects you might want to include.  These could include buttons, bones, metals, etc.  If you can, find them already precut into distinct shapes and sizes.  This will keep you from having to break them with a hammer/saw later. Choose a wide variety of colors and shades.  Even if your work will be mostly monochromatic, you will want varying values and ones. Choose tesserae that goes with the art you wish to create.  For example, if you are doing a beach scene, it might be best to use a wide variety of shells.  If you are looking to recreate the Brooklyn Bridge, you might want to look into metals. Tables, planters, birdbaths, or walking stones are just a few ideas to use.  You can make almost anything work as a base.  Wood, terracotta and/or glass are usually what people use as a base.  Make sure the base is strong enough to hold the weight of the tesserae and grout. Choose bases which have large, open spaces. If there are numerous crevices on your base, it may be hard to create mosaic pieces which will fit. Find patterns for free online or use your imagination and come up with your own design.  For a first time project, you may wish to work with a simple pattern.  This could mean a house, a fish, a landscape, or anything else with simply defined shapes.  After you work on a simple pattern, you can graduate up to more complicated projects. Print out the pattern to the dimensions needed.  Begin transferring the design, first using a pencil.  This will allow you to adjust for mistakes and inconsistencies.  Once you have a rough outline with a pencil, go back and trace over the lines with a pen or marker, depending on the type of base.  If you are using a basic design, this might be the time to add in details that are not present in the original design.  If the pattern is extra large, do it in sections.  Remember this is a rough outline, which can be changed when you start to lay down specific tiles. If you are using an original design, simply draw it directly onto the base.  However, don't over plan.  Spontaneity is key to creating artistically stunning art.
Select the material you wish to use. Pick the foundation to adhere the mosaic tiles on. Find a pattern you wish to make into a mosaic. Transfer the design to your base.