is a total order on S.
Any total order is reflexive, antisymmetric, transitive. Therefore (S, form a poset.
Again as is a total order any two elements of S are comparable. Therefore
we can order the elements of S in such a way so that they form a chain
(say).
The corresponding Hasse diagram is drawn in this convention using the undirected line, the relation (hence, the ordering of the elements) is read from the bottom up.