Same as the root position and the first inversion, we’re interested in which note is on top of the other within a chord. Just like in first inversion, if we have a triad, say A-C-E, its second inversion will be 5-1-3, so E-A-C. That’s the first, the third and the fifth degrees of the scale, but in a different order. Basically, what we do is throw the first two notes with the lowest frequencies higher to the register and use the note with the slightly higher frequency as the lowest note. It’s just a circle. ACEd it. (Not my fault, musicians aren’t really known for their good sense of humor…)