Adding stored procedures to EF when doing code first (For already Existing but forgot to add Store porcedures warlier)

protected override void OnModelCreating(DbModelBuilder modelBuilder)

        {

            modelBuilder.Entity<yourEntityName>().MapToStoredProcedures();

        }

This above code can be used while creating code first approach EF.But what is the process to access Stored procedure for already Existing Entity model?Please Help.Thanks in advance