This is a good question!
Here is a link to lessons that Don Schiff put together.
http://nsstickist.com/nstech.htmThere is some very good stuff here but I don't read music so I stopped after a few lessons.... Sorry Don...
I wish he did videos of each of these lessons...
Here is how I progressed...
After doing the first few lessons from the link above, I had a general understanding of the shapes or chords. (major vs minor shapes and inversions)
After that I started to learn songs. I like to sing so I went with singer songwriter type songs so simple bass (mostly root notes) on the left hand and chords on the right hand. The hardest things was trying to find the best chord location/inversion. Since there are limited number of notes I can hit, I sometimes had to drop some notes and choose the ones that best support the song. (Check out my learning Purple Rain video
https://youtu.be/rJqS3kh0eHI)
I found the chords online and also used youtube videos of people teaching on the piano/keyboard. I found that the note choices on the piano translates well to the NS/Stick.
My next step was to start learning the guitar ballads I love so much... I learned the melody line of Steve Vai's For The Love Of God. I also found the chord progression online and play the bass part with my left hand... again... super simple mostly root notes.
From here I finally started to get the hang of scales and started to see the patterns on the fretboard.
As far as the actual tapping method I learned the most from Greg Howard at the seminars I attended. Learning to move my hand and get the energy into the string. The three finger method really forces you to get the most dynamic range. However... I played bass for many years so I use four fingers on my left hand. But it doesn't matter if you use three or four, the movement of the hand is essential to best tone. (I recommend everyone, even if you have been a long time stick player, to take Greg's intro class at a seminar. I've taken it twice and learned something new each time. Step forward... step to left and right... Where is the body?... If you've taken the class you know what I'm talking about...)
BUUUUUT.... The NS/Stick has many differences as the stick with string spacing, damper on/off, tuning, etc... which lends itself to new techniques.
Don has the slap and strum (in the same motion). You can do more finger picking. I came up with the technique of slap/pop bass playing using thumb and index finger on right hand while the other fingers play melodies. Don also uses a small carabiner in his left hand for slide sounds.
You're only limited by your imagination.
I think at the end of the day, learning songs is the best way to learn...