A pebble pavement should be reserved for places where something is called for; where care and skill in design and making will reflect the importance of a particular location. It should be a focal point, but always placed where pedestrians can choose to walk around it. Given the choice, most of us prefer our everyday walking to take place on a smooth surface, so that we do not always have to look where we are going.
A pebble mosaic is always influenced by it's surrounding, associating well with natural stone paving slabs and bricks, but visibly uncomfortable in association with concrete and asphalt because their lack of natural partitions presents too great a contrast in texture. Equally difficult surrounds are gravel and grass, which will tend to invade from the edges; and the abundance of trees will present problems of dampness and leaf-fall. Ideal locations are open spaces where wind, sun and rain act as natural cleaning and drying agents.
Designing should never be done in isolation. The context is everything. Become familiar with the space, its colors and textures and materials; and take time to learn the resonances it holds for other people. All this will give you a context in which to dream and come up with the kernel of an idea. Then you'll need to ask some more precise questions.
How to Decide on the Size of the Motif
Deciding the size of a motif or pattern needs care. It's difficult to get a clear impression of some patterns unless they are big enough. You can manage to make a daisy look like a daisy with just a few long pebbles for petals and a round one in the center. But try a bird, and you need much more information for it to be at all recognizable. Even a small bird like a robin will be most effective if you actually make it the size of a chicken.
Drawing Your Design
Making a drawing before you start is invaluable. It's not a good idea to improvise at this stage; save that for the pebble details. If you draw your design to the actual size that the mosaic is going to be, you'll be able to test whatever the pebbles you intend to use will fit the design.
It's a common mistake to try to make a design which is too small for the size of pebbles to hand. Consequently, you end up searching for smaller and smaller pebbles to realize the idea, to the point where it gets impractical and frustrating. If your design is larger than the largest paper you have, you will need to make a scale drawing; 1:10 ratio is the easiest. At this scale you can sketch actual pebbles on you drawing, and you will be able to tell whether the pattern is the right size.
Drafting up to the Actual Size
From the scale 1:10 scale original, draw a grid of 1 in (2.5 cm) squares onto the design. On another piece of paper, as large as the finished mosaic, draw a grid of 10 in (25 cm) squares for every 1 in (2.5 cm) of the original. Then systematically copy the lines of the original to each large square.
What Materials Will you Need?
The pebbles that are available will greatly influence your choice of design. Their precise quality will suggest certain themes. Very marbled quartz, large and lumpy, is my favorite for fish scales. Spotty granites are wonderfully suggestive for a bird's plumage. Hard-to-find green stones are effective for patterning of a frog or snake. Practicalities will often dictate a choice: whether it is possible to obtain enough stones of a certain type to be able to carry out the idea you have in mind.
What Will Your Mosaic Look Like?
How much of the mosaic can you see? This depends on where the viewer is standing: at ground level, on a balcony or on a tower. A large mosaic design can only be seen as a whole from a high perspective. On the ground, you can only take a radius of around 2m (7 foot). Try it! The rest is out of focus at the edge of your vision, so the only way to view each part is by walking over it. Designs for large mosaics must there be considered from both the overall perspective and the smaller focus within it.
If a mosaic will be seen from a variety of viewpoints, it should not be designed with a definite top or bottom. Patterns which twist and turn, with a variety of interest from all directions, are to be aimed for.
Make Sure Your Mosaic Fits the Environment
Color is so important! In a stone-built environment choose pebbles that tone with the color of the predominant stone: warm tones for sandstone, gray and white limestone. Try to get the mosaic looking as if it always belonged there. Touches of "foreign" material for contrast and highlights are OK, but the general effect should be one of a more detailed and refined texture than that of the environment. The general rule is to try to use local material for most of the mosaic.
There's no point in being too subtle here; go for maximum contrast, bright colored stones and interesting materials. And make it strong: there's bound to be a lot of heavy use, vehicles and pollutants.