People often think that mosaics are made by pressing the pieces (tesserae) into a background medium, such as cement. This is one possible way, but the most widely used techniques are the direct method and the indirect method.
With the direct method, the tesserae are stuck firmly into place to a backing surface. When the adhesive has dried, grout is applied to the mosaic to fill in the gaps. This is the technique demonstrated in my house number plate project 1.
The indirect (reverse) method involves sticking tesserae face down on to a temporary surface (such as a sheet of brown paper) with a water-soluble glue (e.g. wallpaper paste). When set, the entire mosaic can be taken to its final site and pressed into a bed of adhesive or cement. When it is set in place, the paper can be soaked off and grouting completed from the front surface.
The indirect technique is very useful, for example, where it would be difficult to spend time making the mosaic in its eventual location. This might be the case for a large outdoors mosaic, or a wall mosaic high off the ground. More about this method...
Another way of using the indirect method would be to make a paving stone (where a flat surface is needed). The tesserae are stuck down to paper as before. When dry, a mould is placed over the mosaic and filled with a mortar or concrete mix, to create a slab containing the mosaic (still face down). This is given plenty of time to set and then released from the mould. The paper is soaked off and grouting of the front of the mosaic is completed.
the joy of shards Mosaics Resource