What a great idea ! Perfect for when you want to carry your laptop in a backpack or bigger bag, or if you want the protection but not the hassle of a huge laptop bag.
I think there are two ways to go about making one of these. The first (and I think what's been done here by multidimentionality) is to actually knit the pouch to fit the laptop. This allows you to choose colour and pattern.
The second (and what I ended up doing for mine) was to take an old sweater and sew one together. I found (bought) a nice medium grey one at the local thrift store, washed it and cut off the sleeves. Then I opened up the sides and shoulders and took off the neck. Essentially, this left me with two big squares. On the wrong side of on of the squares, I placed the laptop in the center, folded the bottom part up and over the laptop so that the sides met and the laptop was just covered. Then I measured how wide I wanted the pouch to be. Still working on the wrong side on the square, I attached a hot pink satin lining. Next, I pinned the sides in place and machine stitched the sides to make the perfect sized laptop pouch. Turned the pouch right side out. The top part of the square I folded under a bit and then down to make the flap. Attached a button to the outside of the pouch and pushed it through the flap so that it closed.
It doesn't look exactly like the one in the picture, but I still get lots of compliments on it and it was a great way to use an old sweater.