Learn 2 ball columns, then to juggle them as low as possible.
Once you have that practice juggling just the ken on its own so you get a feel for how it rotates.
I guess you could practice stringless or with a long string at first then practice throwing lower and lower until you can do it with the usual string length.
Also with the big cup juggle (big cup - juggle - big cup) think, knees - no knees - knees i.e. don't bend you knees whist juggling but do on the catches
