I learned it off of 1. a pro Guitar player and 2. a piano teacher.

I am using the G Maj Chord throughout.

Here are the easy steps.
1 Pinching = picking the E Bass with Thumb and AT THE SAME TIME, the 1st finger any one of E/B/G Treble strings.

LOTS OF PTACTICE :)
2 Droping = While Pinching - drop the finger out but continue the thumb. LOTS OF PTACTICE :)
3 Alternating between Bass and Treble = Thumb any Bass string but try to get on the Bass E, after the string sounds pick any treble string E/B/G. Repeat - try to keep this even giving each note equal value. Sounds like Thud - Boing - Thud - Boing etc., LOTS OF PTACTICE :)

4 Graduation :) A finger drops in for a tune hehe

A brief explanation - the idea is to create both a melody and a backing on the same Guitar. Fingered notes are the melody, Thumbed the backing. After you get good at the basics above - you'll be ready (this step is really easy) to 'drop in' That means Thumbing the E string evenly - Thud -Thud -Thud -Thud.... and as your thumb goes off on it's own : add a finger on any treble string.

5
Using the System and Variation etc. The finger notes can fall 'on' the beat - ie at the same thime as the thumb note or 'off' the beat. AND ANY WHERE IN BETWEEN. Here lies the skill in finger picking.
6
Alternating bass, lots of students try at first to jump into this - my experience of teaching( what I used do for a living) DON'T. When the basic Tumb routine is automatic, it is easy to adopt that to a 'cross' string style.
Cross Tumbing.
In a G Chord Thumb THE D string (a nice and I noticed popular habit is to allow the Finger to pick any string in between these notes - kinda balances the hand) NEXT Thumb the G string. Just repeat this over and over untill automatic.

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Mr Thumb gets a PH d
Advanced routine for the thumb G maj
1
Thumb the D string,
2
BRUSH the entire Chord = REQUIRES THE THUMB TO MOVE BACK UP TO GET ALL 4 BASS STRINGS ***************

Thumb the G string
4
BRUSH the entire Chord = means you gotter reverse the thumb to get the Bass G note as well the rest of the chord.

Playing a melody line over this is a kinda wild - sometimes Travis Pickers 'think' you are just a bad Travis Picker (Grin) but you'll soon learn that the system is vastly superior to Grade School Travis 'twiddling'. Listen to Blind Lemmon and as many of the old Acoustic Blues Players as you can. It's all in there :)

After the Phd, doing a boogie as well as a melody is a piece of cake. Heard a wiz once pulay 'In The Mood' with both a boogie backing and the melody on top ( 1960's)

Enjoy