It's not really as difficult as it looks at first, actually. It's just roll for Rune, roll for Rune Effect. And you're done.
Arwyddlun is a 5th level Human Unlettered. As he's the party tank, he wants runes that suit that roll, and chooses Sterkur and Kolo. He then rolls a 1d6 for Sterkur, getting a 3, and a 1d6 for Kolo, getting a 1. This results in his runes being Exhu instead of Sterkur and Queta instead of Kolo. Arwyddlun follows that up by rolling a 1d6 for Exhu and a 1d6 for Queta to see what Symbol he drew. A 1 for Exhu gets him the First Symbol rune effect, while he has to reroll a 3 for Queta, as the Third Symbol rune effect is too high level for him to use. His reroll is a 2, and so his final carved rune is Exhu's First Symbol combined with Queta's Second Symbol.
The events below are exactly the same as the text above, but laid out more programatically:
Sterkur -> rolls a 3 -> gets Exhu -> rolls a 1 -> First Symbol rune effect.
Kolo -> rolls a 1 -> gets Queta -> rolls a 3 -> Can't use Third Symbol (too high level) -> rerolls a 2 -> Second Symbol rune effect.