Hey Sesh.
On a pad, High jump installs (828) are kind of difficult to input fast enough to get the JI. But for the normal JI combo, just press 8 between 5S and 5HS. It's fairly easy to do, just practice with different timings. You want to be pressing the button fast enough and doing the gatling so that the jump doesn't come out.
For Fafnir IAD combos, it's usually 2S -> 5HS -> sj.IAD -> j.p j.p -> j.s/hs -> SV
I bolded the things you had wrong in the combo. j.HS after the j.P only works on people with bigger hitboxes. Axl, Sol and HOS (at very, very specific heights, just go with j.s), Slayer, Pot, Zappa (i can't remember if it's consistent or not), I-No, sometimes Baiken, Ky, Faust, etc.
However I recommend just using j.S on everyone until you have time to practice it for yourself and figure out what you can do on who.