It's not very dirty and it's definitely not a secret, but I'll let you in on it. I don't know anyone else who does wood burning (though I know there are plenty of you out there). I started doing it a few years ago when I moved to Waco and didn't know anyone. It evolved into a fun Etsy business (which is partially why I started this blog), and now I usually just do them as gifts.
Sometime they're done by hand with a cheap wood burning pen (top C), and sometimes they're laser cut (bottom B). I started getting them laser cut to cut costs so I could get more Etsy buyers. I likened it to illustrated cards. Handmade, but mass produced. You can pretty much get anything laser cut if you vectorize it. This allows for more detail in the art (not that this B is a good example of detail). Of course the benefit of doing it by hand is exactly that—it's produced by your very own duck-watching-whisky-drinking-grandpa hands. It's harder to do and it looks less polished. Luckily rugged is so hot right now.
I'd love to improve my work. If you know of better wood or a nice pen, I'd like to know about it. I'm just an amateur, but I hope this inspires you to give it a shot. It's quite fun and nothing beats the smell of burning wood.
RESOURCES: You can get wood planks from Michael's or Hobby Lobby for about $7-$14, depending on size. That's also where I got my $12 wood burning pen (similar to this.) Most cities have a laser cutter, it just takes a Google search to find them. They might even vectorize your art for an extra fee. I've paid $12-$25 for laser cutting on a 12" plank. I recommend Texas Tape and Label if you're in the Waco region or MakeATX if you live in Austin.