What DO you use it for?
Some people use it to precisely align stickers, stamps, embossed details, etc. This past weekend I made a "pop-up" card which had lots of folds and I use my centering ruler to mark folding lines on cards. I could have certainly used a regular ruler, but it just made it easier for me.
SO, if you'd like to make one, the basic idea is to mark "ZERO" in the center and move outward on both the right and left sides.
Here's what I did:
1. I found a ruler. I like clear the best. This one is by Westcott but it's very grimy--with ink, sticky glue and acrylic paint.
2. I grabbed my Best Cleaner Ever. It's GREAT for removing all sorts of gunky stuff and it's 100% safe for hands and has no fumes! Just spray and wipe off. Rinse with water.
What to use? The FASTEST way would be just a permanent marker.
Letter (number) stickers would be good & easy.
I decided to use my label maker beacsue I have one and rarely use it! (I got it with lots of spools very cheap at Goodwill!)
Then I decided to use color so that the same distance on either side would be the same color. Makes it easier to use and I had so many colors!
Anyway, since there's so many more numbers, I decided to use my computer and printer. I opened a document. Changed the "rulers" to metric and typed the digits, aligning them with the ruler. Then I printed on a piece of paper.
I took a strip of WASHI tape, and stuck it to a cutting mat. I sliced it in half and applied it to the printed paper. (I use this technique a lot to be able to print on tiny scraps, etc. You can see it in the Printing on Vellum Video.)
BTW, here’s an old trick for using a ruler with stickers....
1. Just overlap the sticker onto the edge of the ruler. If you use a centering ruler, you can put the center letter at zero and move outwards. (Not necessary but ...)
2. Then tip the ruler's edge so the stickers adhere to the place you want them to stick. Press them onto the paper and roll off the ruler.
Another trick: Make sure things are very crooked and then you don’t have to worry if you’re centering correctly and if they look a little off!