Hi,
You don´t expect that we design the complete charger for you, do you?
So yo could divide it into two main problems:
1) step up voltage from 6V to xxxV to charge the battery
--> look for power conversion. especially "step up converter", or "boost converter". Done many times before. All information already available in the forum. But for sure there is not "the one and only" solution, there are many possible solutins.
2) build a charger circuit which takes care of battery voltage and adjusts charging current.
--> look for charging circuits. It depends on battery chemistry. Charging techniques. Integrated solution or DIY. Again many possible solutions.
Klaus