Hi Bro !
It is to be done carefully. Please dont attempt at home, you need a good drill press, nice hard bits and an experienced drill operator to achieve this without damaging the discs.
Discs are not that hard you know.
Ist take the disc out
Put disc on chart paper and cut a round drawing chart paper of size of the disc, draw pattern on chart paper with help of compass and other stuff from the geometry box of a kid, that you want cut on to the disc.
The pattern should not consist of more holes than 16 (I was scared to have ruined the disc with more holes - stess buildup during hard breaking and increased probability of cracking as some people pointed out at the workshop. I kept the dia of holes 4 mm as too big is not very comfortable in terms of weakening the structure of the discs.
I made pattern in only 2 concentric circles with 8 holes in each one in a way that distance between the holes of two circles were at the farthest from each other but still in the range of the disc pad contact area.
Put the paper-die on the disc and make marks with a permanent marker pen.
Let the holes be drilled slowly with just enough pressure to not to vibrate the disc.
The disc should be held in the place tightly to prevent damage due to possible vibrations.
Last but not the least, my discs were old and breaking efficiency was below average and some mechanics asked me change the discs and pads, but I took it as an opportunity to experiment with the old discs and did this mod.
Now I am so happy with great breaking power of these discs that whenever I change my discs to new I will use pre drilled discs only.
They keep the pad powder out, keep the pads clean, get cooled down faster and somehow the combined effect on breaking efficiency.
Thanks & Regards.
Sandeep