Remember how earlier you center-punched the supports accurately? ...at reliable increments of 12"? Now it's time to put them to the test.

Find your wall

Once found, draw these lines (in pencil! lightly!):

Vertical (and plumb) line down the center of the left stud. Vertical (and plumb) line down the center of the right stud.

Those lines should both be the same distance apart the whole way down . If not, correct it. And, they should be the same distance apart as the center of your keyholes are apart with the shelf put together. (if you're off 1mm somewhere it's probably not a huge deal - the screws can flex a tiny bit.)

Mark for Screws

It's probably best to do this in order if you're feeling queasy about this.

Center-punch the top-left screw location. Draw a level line from the first punch to the right line. Center-punch the top-right where the lines meet.

Now, you have your top keyhole screw locations. Don't add screws yet.

Mark the rest of your keyhole screw locations on the wall. You used proper increments, right? Using the ongoing example of 12" increments, here's a reliable way to centerpunch the remaining holes:

Have a friend hold a measuring tape where the 12" mark aligns with the top centerpunch. You center-punch at each 12" increment after that. (Stop when enough keyholes have been accounted for, or you run into the floor - whichever comes first.

The bottom two centerpunches. should be level, or at least within 1mm.

Screw in the Screws

Find proper screws for this. You want them to be strong and long enough to hold the shelves up, without hitting any wires or plumbing.

I used 2" decking screws (see sketch above for how I determined that). Consider what's lurking behind your drywall. Maybe if you have a monster back there you might want to use some extra long screws in some unthinkable abundance.

Mount the Columns

You'll need to tweak the screw protrusion a few times to get it right, but you'll know when you get that soft click of the column keyholes locking in place.

It's the same satisfying feeling of snapping two Lego bricks together.

Install Shelves

If they're snug, great! But there's a pretty good chance they won't fit, which I'd argue is the right way to err.

Pre-finish, they fit, right? Then, the good news is that you don't have to scrape away any wood; only the finish. And likely, just the columns.

The joinery contributes to safe scraping-away-of-finish without being noticeable. Start with the columns. Scrape off some of the finish that got in the groove area.

In the end, you'll have a snug-fitting shelf that you can wiggle out if you want, but will not fall out on its own.

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