How does one get an atar of 95+ or 99.90???
In a nutshell, be good.
Now to actually go about doing that imo you need to do three things
1. Make the conscious decision that you want to be good
2. Work out a good plan to be good
3. Carry out said plan
If it doesn't work go back to step 2, but then again, if it doesn't work it was a shit plan
In terms of specifics, you have a whole forum full of posts from people who have been through the HSC, and a whole internet of almost every resource you could ever need. Get researching.
More or less, make a plan, and make sure it's legitimately a good one. At that point, it's your responsibility to follow through.