Using the journal is a matter of wrapping the different context changes, being each mount, each modification (transaction) and each changed buffer to tell the journalling layer about them.

Here is a some pseudo code to give you an idea of how it works, as an example.

  journal_t* my_jnrl = journal_create();
  journal_init_{dev,inode}(jnrl,...)
  if (clean) journal_wipe();
  journal_load();

   foreach(transaction) { /*transactions must be 
                            completed before
                            a syscall returns to 
                            userspace*/

          handle_t * xct=journal_start(my_jnrl);
          foreach(bh) {
                journal_get_{create,write,undo}_access(xact,bh);
                if ( myfs_modify(bh) ) { /* returns true 
                                        if makes changes */
                           journal_dirty_{meta,}data(xact,bh);
                } else {
                           journal_forget(bh);
                }
          }
          journal_stop(xct);
   }
   journal_destroy(my_jrnl);