I've been mashing it since cs2 dropped and only started to get it consistently recently.
First of all, after the j6D-D you need to Dash 6C. Its a slight delay after the D follow up, then you input 66C as you're about halfway down the chain/ approaching the ground. It helps to hold the C button at first.
It's rather tricky so watch when they tech to figure when you need to adjust your timing.