Starting from the bottom of the tree would be the pot. Having the right pot can make the difference in how your bonsai tree is perceived and give the impression of a real tree. By having a pot too large, your bonsai tree will look more of a plant in a pot rather then a full grown bonsai. Usually you’re training a bonsai tree to make him look like a real tree but in miniature. So in resume, the pot must be less massive then the tree.
Moving up would be the soil, or covering the soil. Some bonsai enthusiasts prefer to cover their soil with moss. The Kyoto moss is popular because it is a fine bright green moss. Moss can be grown on a separate plate and then place over the soil once you have done a re potting. However, some prefers to sprinkle the moss spore over the soil once they have re potted their bonsai tree. Moss needs humidity and shade to grow.
Next are the exposed roots or in Japanese they are called "Nebari". This is one of the fundamental elements of a great bonsai tree. The exposed roots will add a few impressions to the bonsai. First they will give a sense of a mature tree very well anchored in the soil. It will also help the illusion of a perfect trunk by starting the taper of the trunk.
The trunk, needs to have character and not look like a stick in a pot. This is why, as previously mentioned, exposed roots are important. At the very bottom of the trunk are the roots, and when they are wide, the trunk will usually start wide and will taper as it goes up. On a mature tree, a great trunk is not a cylinder going up, but you can see some taper from bottom to top. Trunks can also have many forms, depending on the bonsai form you wish to give your tree.
Last would be the crown. The crown is composed of two elements. First being the branches and second being the foliage. Branches need to be in proportion and well detailed. The proportion of the branch is considered with the overall of the tree. Short and multiple internodes are essential for the illusion of a matured tree, while staying in proportion with the rest of the tree, being roots, trunk and pot.
Next is the foliage, the leaves must also be in proportion of the tree. Leaves can be reduced in size depending on the species of tree you have decided to make in bonsai. The possibility of reducing the leaves size will dictate the size the bonsai will have. For example, the Norway maple produce very big leaves, so if you want to make it in a bonsai you would need to make it a big bonsai since a 15 centimetres bonsai would have leaves half it size.