NiCd batteries are charged at constant current. (Lead-acid batteries are charged at constant voltage.)
An easy way to make a constant current charger is to use a relatively high voltage and add in a series resistor to limit the current to a safe value.
simulate this circuit – Schematic created using CircuitLab
It's generally safe to charge NiCd batteries indefinitely at 1/16 of their Ah rating. For example, if we had a 1.6 Ah battery pack we could charge at 1.6/16 = 0.1 A. Putting a 60Ω resistor in series with the 12 V source would limit the current to 0.1 A when the battery is at 6 V. It will decrease a little as the voltage rises (good) but it would be 0.2 A if the battery were completely flat.
You can work out the power wasted and the required power rating in R1 using \$P = I^2 \cdot R\$.