Transforming undirected maximum spanning tree into directed augmented network
Given an undirected tree, pick any node (it doesn't matter which one).  Call that the root.  Now do depth-first search (or breadth-first search, or any other kind of search) starting from the root, traversing only the edges of the tree.  That will visit all the vertices and edges.  When you visit an edge for the first time, it will always be in an "outward" direction, so now you know how to orient it (how to set its direction).

Work through a small example if this is still unclear.