The basic stroke is a boom-shukka where each half of the word lasts for the same amount of time.
You hold your hand as if you were hitch hiking, with the fingers curled in and the thumb stuck out.
For the “boom,”, move your whole hand down and have the nail of the Index finger thump a string, usually the 1st, 2nd, or 3rd. Don’t flick your Index finger out; it’s a hand movement, not a finger pluck.
For the “shukka,” bring your hand back up and do the downstroke for another “boom,” but this time also twist your wrist and get the thumb to move into the instrument and catch on the top string (the 5th string of the banjo). The Index fingernail on the 1st string sounds the “shuk.” Now you stop the hand going down and twist the wrist back the other direction to pull the Thumb off the top string and make it sound the “ka.” The Thumb doesn’t move in relation to the rest of the hand. The entire motion is in the wrist.
It’s an easy natural movement that sounds very nice. If you like that sound, you’ll want to study up on Frailing or Clawhammer style on a banjo. There aren’t many books on it for ukulele or guitar.
If it sounds good, it is good.
There are also lots of different sound effects you can make such as pull-offs, hammer-ons, slides, chokes, chimes, and animal sounds.