The letters themselves were constructed from a series of straight lines incised across a single stave(a.k.a druim). These were usually written using the edge of a stone as a stave from the bottom > up they would then run right over the tip and down the other side (they were read from left to right)...this sometimes makes ogam very difficult to translate.
Branch ogam was usually carved on a branch of the specific tree it is related to, the way it is inscribed is in the same vein as the stave version. This is the most commonly used for divination purposes, for those of you that wish to create your own ogam set, gently collect a branch from the specific plant, stave the end of slightly and carve into the exposed wood. These would then be cast or picked from a bag. If they are cast the relation between the overlapping twigs and the ogams meanings are to be integrated.
Finger ogam was perhaps the most secret, because certain fingers related to specific letters/associations druids could pass secret messages by touching anothers hand in a certain way or by holding an object.
If you click on any of the tree names you will be taken to a description of the relevant tree you will then need to press the link provided to return here.