After I double the sewing thread and knot it, I go through the button 3 or four times.
The pictures below show how I finish it off.
In the first picture, I take one short stitch.
In the third picture, I wrap the thread around the needle 3 or 4 times.
Then I do the short stitch again 2 times to finish it off.
If I still don’t feel secure about it, I wrap thread around the needle again and do the short stitch.