This is my friend's battery pack and I wanted to surprise him by making a homeade charger. How would i go about that? Is it possible? Is it expensive? I have basic soldering skills. Thanks so much.

 

The battery type is Nickel Metal Hydride. I would try to find a commercial unit since designing and building a charger is a non-trivial task even for an experienced engineer. There is a great deal of good information on batteries and chargers at Battery University. You should probably start there.

Battery University Homepage

Here is an excerpt from the article on NiMH Charging:

The charge algorithm for NiMH is similar to NiCd with the exception that NiMH is more complex. Negative Delta V to detect full charge is faint, especially when charging at less than 0.5C. A mismatched or hot pack reduces the symptoms further.​

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NDV in a NiMH charger should respond to a voltage drop of 5mV per cell or less. This requires electronic filtering to compensate for noise and voltage fluctuations induced by the battery and the charger. Well-designed NiMH chargers include NDV, voltage plateau, delta temperature (dT/dt), temperature threshold and time-out timers into the full-charge detection algorithm. These “or-gates” utilize whatever comes first. Many chargers include a 30-minute topping charge of 0.1C to boost the capacity by a few percentage points.​

If you think this is a trivial weekend project, then you might need to think again.

 

Charging such a battery QUICKLY is a complex process, as has been explained. But a lower voltage slow charge can also be used. as the battery is 1.2AMP HOURS, calling it 1200 milliamp hours sounds more impressive. A float charger set at a regulated voltage to deliver 100 milliamps at the rated voltage could recharge it in 12 hours.
Unfortunately most RC car drivers want a much faster recharge time so the more complex charger will be a better choice.

 

Those are normally charged with a constant current supply. End of charge can be detected by a voltage dip or something, though I find that chargers rarely do that correctly. The more reliable method is to detect temperature rise, because once the battery is full, further charging just creates heat.