I have to credit my mom for this one. Here in Portugal we also cure/flavour olives.
This is simple but delicious.
Buy a large jar of green olives, still with pits.
Open the jar and place the contents in a bowl, including the brine.
Place a layer of the olives back in the jar.
Sprinkle liberally with cumin seeds, finely minced garlic, and piri-piri sauce (if that's not available, you can add some whole bird's eye chilli).
Repeat layers until the jar is full.
When you're done, pour the brine back into the jar.
Close and store in fridge.
Easy and yummy.