Here is my leather ring, it was my second attempt, I ended up trimming the first one too short, and it didn't fully protect the pad of my thumb, so it has been modified for use by my wife and son.
The pen marks are from ideas about angle and pad width and length. I have about 700 arrows downrange with it now and may narrow the pad a little.
The leather ring took be about 20 minutes to make, and once the pattern is correct they will take about 5 minutes to churn out more.
The leather ring.
Hook the thumb below the nock of the arrow.
Use the index and middle finger to reinforce the thumb
The index finger will keep the arrow tight against the right side of the bow.
Once full draw is reached the thumb is relaxed or flicked and the string glides off the pad of the thumb ring.