There are a bunch of videos of guys doing that floating around. Try youtube. Look at how they are doing it, emulate it and practice, practice, practice. Like anything else, as your technique improves so will your sound. Keep in mind that the shape of your thumb is going to effect the sound of your upward "thump". Personally, I have really skinny roundish thumbs

(as opposed to flatter thumbs with some meat between the nail and the edge of the finger) and have never been able to achieve a uniform sound in my up and down-strokes. All flesh on string on the down-stroke, lots of nail on the up-stroke.