Go into training mode and have the a.i either jump or super jump and practice just doing j2a>66>j2a. It took me quite a bit of time but once I figured out the timing the working on that specific part the combo(s) became much easier. Sometimes it's best to concentrate on the specific part of a combo that gives you the hardest time.
With hakumen I would say people have the hardest part for people to complete with basic hakumen combos is j2a>66>j2a>jc, and SJ2A>2c>SJ2a. For things like that you need to isolate that part to understand the timing of it rather than just mashing buttons hoping that it'll come out. When you can perform that specific part, then you incorporate it into the entire combo.
I hope that helps.