Finding max flow of an undirected graph using Edmonds-Karp

Question

I was recently trying to solve a max flow problem on spoj. I saw an algorithm for max flow here, so I applied it but I was not getting the required answer. The reason was I decreased the capacity of edge towards sink in my implementation and eventually it reaches zero and it does not consider the reverse capacity (which many people claimed in online to add two edges one forward and one backward in place of one undirected edge). Could you please help me out from this or tell me how to solve problem using Edmonds-Karp algorithm.