Step one: Paint a board black on one side (my board was about 4 inches x 1 inch x 6.5 feet. Make sure it is a strainght, smooth board without any warping.
Step two: I use Sure Cuts A Lot 2 software linked to my Cricut to cut out the numbers and hash marks for the oversized ruler. I cut them out on contact paper, so I can place the contact paper numbers on the wood and remove without leaving any residue.
Step three: Use a measuring tape to mark off and place the contact paper numbers and hashes accurately.
Step four: Paint over the board and contact paper with a white or cream paint--I just use whatever housepaint I have extra. While the paint is still damp, peel off the contact paper. Be careful not to smudge the paint onto the revealed black paint. If you do smudge a little, don't worry, you can either go over it with a black sharpie or let the brown stain mute it out in the final step.
Finally, start marking the kids' heights with a sharpie! I'll even be able to pull this out for my future grandkids to compare how tall they are to their parents!