Because I get so many questions about this step in one of my published papers. (Well more accurately, my PI gets questions about this step and he sometimes forwards them on to me for an answer). The paper referred to in this guide is this one.
There are two completely different steps to reconstructing maize subgenomes: 1) putting together ancestral chromosome pairs 2) grouping one copy of each ancestral chromosome together into subgenome 1 and the other copy of each ancestral subgenome 2.
Ancestral chromosome pair reconstruction: