Olives are very bitter in their raw state. The first thing you need to do is get rid of the compounds responsible for that bitterness. There are a few ways to go about it. You can use lye, a great method if you are impatient. I don't like it because it robs the olives from some of their fruity flavors. You can use salt, which is so much fun, and the olives come out delicious! I wish I had enough olives to salt-cure a few. Next year...
I am going to use water. It's a safe, easy method, and the olives come out great. The basic principle is to soak the olives in plain water, and change that water every day. Little by little, the bitterness will wash away.
To speed the leaching up a bit, I scored each olive using this tool.
I bought this board a few years ago in a hardware store in Seville. It's made of beech wood, and has four little metal "blades" in each of the holes. Each hole is a different diameter. You place the board over a bowl or bucket, and push the olives through the holes.
You can achieve the same results with a paring knife and a bit of patience, or you can smash the olives, instead of scoring them. Smashed olives are super-authentic, and you'll see them in every bar in Andalucia. To smash them, the only thing you need are a hard surface and a mallet. Also, I highly recommend you wear a pair of gloves. Otherwise, you'll sport very black fingers for a few days.
So once your olives are smashed or scored, put them in water, and change it every day. Rule of thumb is about two weeks for green olives. Mine are pretty ripe, so I'll start biting into them around day 5 or so, just to gauge the level of bitterness. I don't mind if the olives stay a little bitter. Every element, in its just measure, adds complexity to the overall flavor.