My dad has a farm property with an old barn on it so when we were there a while ago I searched for the perfect piece of barn board to make a growth chart for our house. I'm quite pleased with how it turned out.
I wanted to keep the look of the wood pretty rustic, so I didn't do anything to prep it other than wipe off the grime with a slightly damp cloth.
Using a ruler I marked off where I wanted the lines to go (every inch). I decided I wanted short lines every inch, but medium lines every 3" and the longest lines every 12" to make each foot. This way it looks like an old metre stick.
Once I had the numbers the way I wanted them, I simply laid them on the barn board in the correct spot (at every 12" line) and with a really sharp pencil, I traced over the numbers on the paper. This left enough of an indent in the wood underneath that I was able to fill it in with black paint.
I then traced over the lines marking the inches with the same black paint.
You could shellac the whole thing once the lines are painted, but as I said, I really wanted to keep this rustic looking. Plus, I wanted to be able to write with a pen, pencil or marker the names and dates that my kids were measured. After all, this is supposed to be functional.