Warning: As the sign says, these wooden feet tend to slide on the floor, so make arrangements to prevent that. Actually, ladders are all dangerous. I cringe when I see them on "funniest home movies" because I know something horrible is going to happen.
I used a clear 2x4 ripped in half for the sticks and 3/4" plywood for the rest of it.
The treads are 12" apart vertically and the holes are 10" wide.
The ladder is 16" wide. I would make another with exactly these dimensions.
This plywood is cheap so there are voids, and some of the treads seem weak.
Those will get reinforcing sticks across the bottom of the treads.
Step 1: Cut the Holes
Then I drilled a hole inside that and cut along the line with a jigsaw. Then I rounded off the corners with a router and a quarter-round bit.
If you've got a sawzall you can do a plunge cut and skip the drilling. If you're feeling organized and like routers a lot you can make a template and do the whole thing with a router.
I could have used a stencil to draw the holes but after I drew the horizontal and vertical lines there wasn't much left to draw.