First at all, sorry if my english is bad.
I own the imax b6, and it´s a good charger. Look up, tere are many clones.
If you want to build it at your own, you will need to control the charching voltage of each cell and its charging current. The little conector is used as charging port and test port, and the big connector is used to know the total charge. As I said, you must control the charge of each cell indivisually, so if you have the total charge in one cell, you must stop charging it but continue charging the other. It's a little tricky but possible.