If your thumb nail is non existing then you buy a thumb pick and try that but my experience with those is that it cuts of the bloodflow to the tip of your thumb and then you get a numb feeling so for me they only work for one song and then I have to take it of, another way is hybrid picking but you loose the index to hold the pick.

Growing a small thumbnail with a little edge on the left side of the nail ( right handed ) just enough to pluck the strings is for me still the best way.

Herman