GROUTING

Anyone who has ever had the pleasure of mixing cement will have no trouble grouting.
  Buy the finest grout available. Powdered grout is available in grey, white,ivory and brown. It is also available in a variety of colours 'ready mixed' which is fine, but has a shorter shelf life. This can be greatly improved by cutting out a circular piece of paper and laying it on the surface of the grout that is left in the pot to prevent the air from getting to it and drying out.
  Always follow the health and safety instructions on the packet. If you are using powdered grout, you can colour it at the 'dry' stage by adding coloured powder paints and/or up to 50 percent fine sand.I recommend that you stick to plain grey grout nine times out of ten. The grout  delineates the tesserae and if it is brightly coloured, it can distract or even overpower the effect of the mosaic. I occasionally use brown grout if the overall colour of the mosaic is 'warm' and brown seems to be the harmonious choice. Although I don't often use brightly coloured grouts, I have seen them used to excellent dramatic effect, especially in jazzy, abstract mosaics.
  To mix the grout, wear rubber gloves and make a 'volcano' by pouring out the grout onto a flat piece of scrap wood (hardboard is ideal) and make a 'well' in the centre. Gradually add water to the well and mix thoroughly with a small hand trowel. The mixed grout should be of a 'mud pie' consistency - if it is too stiff, it won't flow into the gaps, if it is too wet it will wash out of the gaps when you clean it off.
  Starting on the vertical edges, spread the grout with a 'squeegee' or tile grout spreader. Then grout the top surface of the mosaic, running the grout spreader across the surface in all directions in order to fill the gaps thoroughly.
  Wipe off the excess grout from the sides and top surface using a damp cloth. Sometimes in a direct mosaic, a few of the tesserae are set lower than the rest and so become submerged by grout. Once grouted, reveal any such tesserae using a penknife or other suitable tool. Take some trouble at this stage; it is much harder to remove grout once it has set.
  The drying of the grout is a chemical process and takes about 24 hours. There is nothing to be gained by placing the mosaic lacks some of the shine it had before it was grouted. This is partially due to an unavoidable scum of grout remaining on the surface. In a couple of days, when the grout has set, give the mosaic a scrub with  a liquid floor cleaner using an abrasive sponge pad.
  If you are making a direct mosaic plaque, screw in 'D' rings or mirror plates, available from your local ironmongers, and hang it in a suitable place - somewhere that allows the light to play on the surface.

WHERE TO BEGIN

To transfer a design from an existing template or illustration, place carbon paper, ink-side down onto the surface you are about to mosaic. Trace off your chosen design onto tracing paper and place it on the carbon paper. Firmly trace over the design with a sharp pencil to transfer it.
  If you are a beginner working on a project to, say, mosaic a straightforward plaque such as the Gecko, the temptation is to start on the Gecko's face. In fact, there are three good reasons why it is best to start on to  border first.
  First of all, you will get useful practice at cutting the tiles into tesserae. If a tile accidentally cuts at an angle that is not straight, you can compensate by cutting another to match this angle.

Mosaicing straight lines. Compensate for ill-cut tesserae by cutting other tesserae with angle to match.

   Secondly, when the time comes to grout the finished piece, it is the edge tesserae that take the biggest knocks with the grout spreader. If any tesserae are going to fall off, it is invariably these ones. Those in the middle have their neighbours to support them, the edge ones don't. So if you have  mosaiced the edges first, you will be certain that the glue has well and truly set and that these tesserae are firmly in place.
  And lastly, before mosaicing the edge tiles, the work surface has to be completely clean. If there are any glass shards under the board from where you have been cutting, the board will not rest flat. If you begin to mosaic the edge first, before you have done any cutting at all, there is no need to clear and clean the work surface, it is ready to make a start. Remember to place a sheet of paper or newspaper underneath the MDF panel to prevent it from sticking to the work surface.

MOSAICING THE IMAGE

Having mosaiced the border, you are now ready to start on the image itself. You will soon find that it is a lot easier to mosaic next to tesserae that have already set. So, if you are mosaicing, say, an eye, it is a good idea to initially place the pupil in position and let it set for half an hour or so (if you are using PVA). When you later add the iris, there is no possibility of the pupil being disturbed. This is only necessary on very precise areas such as facial features, where you want to capture a specific expression and the exact placing of a single tessera can be vital.
  Always mosaic the object that is closest to you in visual terms. This is because, however hard you try, the mosaiced line will never precisely follow your pencil line. Imagine you are mosaicing a fish swimming through water. If you worked the opus vermiculatum-- of the background first, this would dictate the outline of the fish, which is obviously more important. Thus, if a fish is swimming through
seaweed, for example, the seaweed needs to be worked on first, then the fish, and finally the watery background.

GLOSSARY OF MOSAIC TERMS

Andamento The generic word to describe the general 'flow' of the mosaic.
Interstices The spaces between the tesserae.
Opus musivum See below
Opus regulatum A Roman mosaic technique whereby regular, square tesserae are applied in straight rows. The result is like a 'brick wall' pattern and was frequently used to fill expanses of background.
Opus sectile When a part of the mosaic, such as a head,consists of only one part then this part is known as opus sectile. If the whole area of the mosaic is covered in this way the resulting effect is more akin so stained glass or marquetry.
Opus tessellatum A Roman mosaic technique whereby regular, square tesserae are applied in a rectilinear arrangement. The resulting uniform 'grid' design was most frequently used to fill expanses of background. You would reasonably expect this to be the most common opus-- for background work, but in fact, because the tessarae are irregular, the rows rarely meet up on two axis. Thus Opus regulation is the most common opus used for background work.
Opus vermiculatum A Roman mosaic technique whereby regular, square tesserae are applied in a row around the main mosaic motif to create a halo effect and emphasize the setting lines of the design. Vermis is the Latin word for worm (as in vermicelli!), so you can think of this as the 'worm' of tesserae that outline the main figure(s) (See White Flamingo) If the opus vermiculatum is continued outwards to fill a larger area then this area becomes Opus musivum, such as in the background of the Gecko Plaque. This is the most rhytmic and lyrical of all the opus and literally it means 'pertaining to the Muses'.
Tesserae 'Tesserae' is a Roman word meaning cube (pl. tesserae). These cubes are the basic building blocks of mosaic. The term embraces diverse materials, including marble, ceramic, glass and pebbles.