A number of participants in OLW7 have moaned about ruining lots of paper trying to get exact placment of their images. I feel your pain. If you're gonna do CAS, you gotta be willing to ruin paper. It goes with the territory. It's unavoidable. It's deeply annoying. It might make you run back to multiple layers and tons of embellishments so you can hide the slips.
DON'T PANIC!
I love the Hitchhiker's Guide. Have I mentioned that? Not relevant, but still.
I need coffee.
Anyway, back to paper. Especially if you are a beginning stamper or even if you're just branching out into more CAS style, it's not always obvious how to achieve precise stamping. You'll absolutely have to pitch mistakes in the recycling bin, and that's okay. Accept that and move on with a few handy tools that will minimize (but not eliminate) mistakes. Let's get started, shall we?
First, the easiest thing you can do to get precise placement is use clear stamps with a gridded acrylic block.

This isn't full-proof by any means. Blocks slip, stamps smoosh, smudges happen. But the grid helps you get even spacing and straight lines, especially if you stick out the tip of your tongue to the right side of your mouth and don't breath while stamping.
Okay, I'm kidding about the not breathing. But the tongue sticking out works. Sometimes. Or not.
Not everyone likes clear stamps, and not all images are available in clear. Let's say you love the feel of wood blocks in your hands as you stamp, but wood is, sadly, opaque. So is rubber. You can still get nice, even spacing like this:

DON'T PANIC! You can do it. All you need is a
ruler (I like this clear plastic L-shaped ruler because it's easy to get right angles with it),
pencil, and
stamp positioner. A gridded cutting mat is a good idea, too, but not essential.

Start by drawing a light pencil line where you want your images to be.

At this point, you can mark the center, or you can use your grid to eyeball it. See how I centered the card on the grid? It's one less mark to erase, but seriously, how hard is it to just erase? Not very.

Put your central image first, and once it's in place, put the ruler down and mark where you want the others at even intervals. It's a good design idea to have the end images run off the card a bit, but not essential.

Next, use your stamp positioner to get each image in exactly the right spot.

DO NOT prematurely erase. You will smear the ink and hate yourself. Go ahead and set it aside to dry, color it if needed, and THEN erase the line. Patience is a virtue.

See? It's not that hard if you get all obsessive about it. Practice makes close to perfect, too.
Note on Stamp Positioners: I bought mine YEARS ago. PSX has been out of business for a long time, but all positioners work the same way. A flat square or rectangle of plastic or acrylic fits inside an L-shaped guide. Nestle the L-shape against one corner of the rectangle, and let the L-shape guide your inked stamp onto the rectangle. Then position the image on the rectangle over the place you want to stamp it, slide the L-shaped guide onto the corner, remove the rectangle, and then stamp onto the paper by sliding the stamp straight down the L-shaped guide.