I did this because I had fairly new disks and I did not feel like buying new ones. Mostly I did this for performance, as it cools the brakes faster and the rotational weight is smaller, making car accelerate and brake quicker.

For the REAR brakes, that do only a fraction of the car braking, I knew I didn't need to concern myself with a lot of braking area on the disk, so I could put a lot of holes in them.

For the FRONT brakes, they do most of the braking, so I wanted to keep as much braking surface as possible, so that's why I put slots in the front with only a few holes.

		Click on photos to enlarge them
First, I used the lugs to divide the wheel into 4 parts, each part will have 4 holes at different heights, and each part had to be precisely symmetrical, so the balance has to stay perfect. In total 16 holes, but that is a personal preference.
Here is a template I used for all the holes, the top one is about 1/4 of an inch from the edge of the disk. Then I drilled, the disk is made of iron and it's quite easy to drill, a good drill will go through it like butter.
Once it was all drilled and I chose a random pattern, but it is a personal preference, the key is to keep it symmetrical so the balance is preserved. Anyway, once it was all drilled, I beveled the holes with this nifty drill bit.
For the final touch, I sand the holes with 200 grit sand paper. I did the front disks of the car and once the drilling and slotting was done, I had them cryo-temped for extra strength. I have been using these brakes since 2009 and they work great!