How can you draw QR codes with a Silhouette? (not create QR codes, assuming you've already got the code)
First, you want to turn on the grid and size the QR code to the grid (hint: the small, solid black squares at the corners are 3x3). It might be easier to set the QR code picture to be slightly transparent - like in the picture below, choose the fill color tool (first icon that looks like a bucket), advanced options at the bottom, then change the transparency. Then, turn on snap to grid and draw rectangles over the black parts of the code. You can also trace the code, but the results won't be as good, especially if you're trying to make the code as small as possible.
Like with writing text, I first tried a cross fill (right drawing), but that didn't work. So I used Internal Offsets. You'll have to play with the offset, depending on the width of your pen and the size of your QR code. At first, my code was too small (top 2 incomplete codes; 2 cm is too small), so I enlarged it, and the bottom code works!