well, good starts are always tricky and traction levels usually vary even in dry because of dynamic weather
But what i suggest is:
Before start, use throttle to light all green led and maybe reds but no blues.
DO NOT FLOOR
Be very gentle on throttle when on first, shift to second before first green led on dashboard goes up, put a bit more throttle shift to third on full green led, put about 60% of throttle and when on fourth floor it and don't use kers before third gear
i also recommend put second gear a bit longer than default and use 0.7-0.6 rear camber when possible
here is an example of start which put me from 4th to 1st on a very short run down to 1st corner
To train starts i do use gp mode "restart race" feature
Basically, try to learn moment when rear wheels are began to spin and just lift. Usually you will go a bit sideways when your car loses traction. From what i learned shortshifting is ineffective, but throttle control is everything.