Draw your picture. Before you create your mosaic, you should sketch out what you want it to look like. For larger forms, try to consider images that would look good blown up, such as landscapes. Even if you are simply trying create a design, you should still try to sketch it out as much as possible, so that you have a template to follow.
Use a free template, so that you can experiment with placing tiles in different patterns and to see if you can recreate your drawing with tiles. Select the "Large" option, so you can get a better sense of how larger forms will look.
Prepare the surface you're going to be laying the mosaic on. Surfaces such as wood and concrete are good. You may wish to lay a piece of paper over top of the surface with the pattern already drawn on top.
Select the tiles you want to use. If this is your first mosaic, you should probably stick with simple square tiles. Once you get more skilled you can experiment with breaking the tiles into different shapes. Lay the tiles out over your pattern to ensure that you have enough.
Spread adhesive on top of the surface and firmly push each tile in place. For larger form mosaics, it's a good idea to use cement, as craft glue and other adhesives may not be strong enough. If there are large gaps in between tiles, you may wish to fill them in with broken off pieces of tile.
Check on your progress by stepping back. Large form mosaics only take their shape from a distance, you will need to continuously gauge the mosaic from a distance to avoid making a mistake.