A ZDD stores a set of sets as explained on slides 140-142 of the BDD chapter. The set of minterms {}, {a}, {c}, {a,c}, {a,b,c} is thereby decomposed as follows (using the variable ordering

a = 1: {a},{a,c},{a,b,c} --> {},{c},{b,c}
a = 0: {},{c}

Note: first split the set {{},{a},{c},{a,c},{a,b,c}} into the minterms that contain a, i.e., {a},{a,c},{a,b,c}, and those that do not contain a, i.e., {},{c}, then remove a.

The procedure is then repeated for all variables until only {} and {{}} occur which are the leaf nodes.