This story was written down in medieval times. If it is based on older oral lore is unknown, but highly likely, as that was the purpose of these stories being taken down by the monks and it is possible that converts who knew these stories were the monks even. I would propose, that if people wanted to use ogham for divination in the most reconstructionist way possible that they would consider choosing one of the following:
Choose a wood like Yew and either plane each of four branches to have five sides and have each branch be an aicme and put the five ogham on each side like a strange sort of dice. Then you would throw those and note which fid came up and how the lots crossed each other or were arranged on your throwing surface.
Make 20 staves of the same wood (or different woods if you are so inclined, a bit clumsy though as they end up different weights), and draw four for doing divinations. At that point they could be thrown or just looked at in sequence.
Choose four fid that represent the possible paths you see for your situation for example a yes, no, maybe and try again later set and select one of those from a pouch to divine the answer to your question.
Additionally, there is the Pregnant Woman Ogham from the Auraicept na N-Écesthat describes incompletely a way of using Ogham for divining the gender of the unborn child.
Pregnant women Ogham, that is, the name of the woman is divided there unless she bear a child previously. If, however, she bear a child, it is the child’s name that is divided there; and if there be a letter over, it is a boy. If it be an even number, it would be a daughter that will be born of that pregnancy.”
If I were to try to replicate this, as it does not tell at what point to divide the name, I would choose a stave for all the letters in the mother’s name. If there are duplicate letters in her name you must choose another stave to represent it and mark it down (using huath for second d in name, etc). Then shake up all the letters in a bag and pick one out of the bag, note what number that letter appears in the sequence of the name and then determine if it is even or odd. Viola you have a prediction of what gender your child will be assigned at birth.
Now this is of rather limited usage. But I can see it becoming applicable in other ways, using ones name or another name to answer other questions. Recall above where we needed to limit the number of staves being used to four? Well you could choose the letters/fid of your name to use for that purpose as well—limiting the types of answers you could have to four or so.
So ends entry one, which is just about layouts based on mythological and historical information. I will delve into various methods of determining the meanings of your resulting readings in future entries, and methods for making your staves as well. I will also do a more introductory post on Ogham.