I'll use x = "processing" as an example. Throughout the example * represents the null character, which sorts before any other character (i.e. * < a < b < c < ...).
We start by creating three groups of suffixes - S0, S1 and S2 - so that each suffix in the group Sk starts at the index 3q + k for some value of q. In plain English:
S0suffixes start at positions 0, 3, 6, etc.S1suffixes start at positions 1, 4, 7, etc.S2suffixes start at positions 2, 5, 8, etc.