There are patterns designed specifically for mosaics, but you do not have to be limited by using just mosaic patterns. If you find a stained glass pattern, you can easily adapt it for mosaics.
The major difference between a mosaic pattern and a pattern for stained glass deals with the spacing between the pieces of glass and the size of the individual pieces.
Many mosaic pieces are used outdoors. Due to the range in temperatures that exist outdoors, mosaics made with large pieces of glass will crack over time. To avoid this, be sure the glass is well seated in the cement and there are no air bubbles under the glass. Another way to avoid cracking is to use smaller pieces of glass. The larger the piece of glass, the more of a chance there will be for future cracks.
Another factor you need to consider is the amount of space between the individual pieces of glass. Depending on the pattern you use, spacing between pieces of glass should not exceed 1/4 of an inch.
To turn a stained glass pattern into a mosaic pattern, you first need to resize it to fit your mosaic mold size. Then cut the individual pattern pieces 1/16 to 1/8 smaller. This will give you a spacing of 1/8 to 1/4 inch. If you would like the look of traditional stained glass, cut the individual pieces 1/16 smaller. Spacing of adjacent glass pieces should be uniform.
If the pattern you have chosen has large pieces of glass, cut these pattern pieces out (1/16 inch smaller), then divide that pattern piece into smaller pieces with 1/32 inch space between the pieces. Using this smaller spacing within the design, where once was a large piece of glass, will convey unity and prevent future cracking.
By using the two different spacings, the overall effect will be a unified piece. Larger spacing will provide definition and the smaller spacing will unify the color.
Another alternative for turning a stained glass pattern into a mosaic pattern is to first resize the pattern to fit the mosaic mold you plan to use. If you are using a clear mold, after wiping down the mold with Vaseline, cut a piece of clear contact paper to fit the bottom of the mold and place it in the mold sticky side up. Then place the pattern under your mold. Using glass nippers make small piece of glass to fill the areas within the pattern. Remember to place the individual piece upside down, when you remove your mosaic piece from the mold the bottom will now be the top. When all glass is on the contact paper, firmly press the glass to obtain good adhesion. Now you are ready to fill the mold with concrete.
If you are not using a clear mold then place the clear contact paper sticky side up on top of the pattern. Proceed as if you were using a clear mold until all the glass is placed on the contact paper (upside down). Then carefully place the contact paper with the glass into your mold.