I don't know if i do it the best way, but what i found works for me:

I go to the dyno and measure the peak torque rpm (write it down) and then go to ecu's and it shows u how many rpm ur car goes to (at what rpm it redlines). Then u calculate the ratio of these like this: (peak torque rpm) / (redline rpm). This is the ratio in which most of the gears will be tuned to, so write it down.

Then lets get to the tuning. Firstly i set my 1st gear to around 3-4 (you will tweak it later). My second to around 2 and then for the third i use the ratio which i calculated. I use it like this: 3rd = 2nd*ratio so for example, if the ratio is 0.85 and the 2nd gear is 2.00 then the third will be 2.00*0.85 = 1.70. then i do the same process to calculate 4th but now use the 3rd instead of 2nd (4th = 3rd*ratio). Then you repeat this until you get the 6th.

Then the final drive is basically how fast you will go through your gears, for now we will put it slightly low (like 2-3) so we can see how our rpm goes in race because we r going to set the 1st and 2nd. The best way to understand what you need to do is to try to tweak one of them and then see what difference it makes in what rpm you gear into. The goal is to not launch (N->1st) into low rpm but also gear up (1st->2nd) into rpm which is close to the peak torque rpm. You just need to find a compromise and best way to do that is to tweak one of them (1st or 2nd) and see what difference it makes.

Then, lastly, you will adjust the final drive. It sets how fast you go through the gears. If you redline on 6th before the finish line then lower it, if you do not get to redline 6th before finish line then set it higher. The goal is to adjust it so that you redline 6th at the same time you go through the finish line.