Class diagrams has nothing do with flow. For more information about realization association
http://en.wikipedia.org/wiki/Class_diagram#Realization.
Are you purposely acting ignorant?
@OP UML Diagrams are divided in two categories. Structural and Behavior Diagrams. Flows are generally shown using Behavior Diagrams => Activity Diagrams,Sequence Diagrams, Communication diagrams, interaction diagrams ( uml 2) etc.
The flow you are trying to show is generally not shown using UML. You can use some tool like MS Visio, if you really want to show something which is not available in standard UML notation and use your own notations.
Good luck.