This should take you about 15 minutes from start to finish, it's a very easy and fun project you can use to get started with a laser cutter. You're going to need the following:

1x Laser cutter.
1x Scrap piece of something the laser can cut. (I used cardboard)
1x Photoshop or similar. (As long as you can enlarge, blur, and adjust contrast you should be fine)
1x Corel Draw or similar. (Inkscape/AI etc will work just as well)

Step 1: Generate Your Code.

The first thing you're gonna wanna do is generate your QR code. I find that qrcode.kaywa.com works really well. You can use whatever you want, but be aware that you're going to have to do a lot more work as the codes get more complex as all the whitespace might not end up being contiguous right off the bat and you'll have to manually edit your code to make sure that it is. I used techshop.ws for mine since that's where I made this stencil. Once you're happy with your code save it out and open it in Photoshop.

Step 2: Resize the QR Code.

Once you've got your QR code open you'll need to resize it so that it's a bit easier to work with. You can do this by hitting Alt+Ctrl+I or navigating to Image -> Image Size. Once you're there make it bigger by about 1000%, make sure the scale factor is an integer. (A multiple of 100%) Also make sure that your resampling algorithm is set to "Nearest Neighbor".

Step 3: Make Sure You've Got Contiguous White Space.

Next you should edit your code to make sure all the whitespace is contiguous. In this code I only had to edit the targeting markers in the corners. I simply turned on the grid (Ctrl+') and drew 45 degree angle white lines through two of the corners.

NOTE: It's not necessary to edit white pixels that share a corner with the rest of the white space, we'll solve that problem in the next step.

Step 4: Round Your QR Code for Easier Stenciling.

Here you're going to blur your image, I used a gaussian blur (Filter -> Blur -> Gaussian Blur...) at about 9 pixels for this size of image. You want to make sure your code is still recognizable but the edges are nicely rounded. After you've blurred your image go apply a brightness and contrast adjustment (Image -> Adjustments -> Brightness/Contrast...). Make sure you've got the "Use Legacy" box checked and play around with the values until you get a good result where the whitespace is all contiguous but the code isn't so distorted that it won't read. If you have trouble reading the code go back and play with the setting on the blur filter and contrast adjustment. Once you're satisfied with your code, save it out and open up Corel Draw.

Step 5: Create Cutting Vectors.

Create a new file in Corel Draw and drag your QR code image in. With you image selected go to Bitmaps -> Outline Trace -> High Quality Image.The default setting should work just fine. Once your image is traced simply remove the fill and set your outlines to hairline thickness and you're ready to cut your stencil. Throw whatever it is you've decided to cut the stencil out of in the laser bed, make sure you've got appropriate power settings and hit go.

NOTE: You might want to draw an outline around your code if you've got a big piece of material in the laser bed and you're not planning on using all of it for the stencil. I used a simple rounded rectangle.