Pick out an animal-fiber yarn which is shrinkable in the wash; wool, alpaca, and mohair are good for this. Make sure it isn’t superwash.
Knit a flat rectangle that covers the width of the laptop (plus room for shrinkage) and is long enough to cover both sides plus a flap. The stitch pattern should have some body, such as garter stitch or a broken rib pattern. I used garter stitch in modular rectangles, but unless you’re using a variegated yarn, the directions in the garter stitch will barely show up.
Place markers at each end of the row that meets the bottom of the case on one side, and again at each end of the row at the other side.
When the first rectangle is finished, pick up the stitches between the markers on one side, and begin knitting upward. At the beginning of each row, both right and wrong side, pick up the edge of the previously knitted fabric and when you come back to it at the next row, decrease that stitch with the one after it. This will join the new strip to the sides of the old one without increasing the width.
Repeat for the other side. You should now have a loose case with its sides joined seamlessly together. Zip it in a net bag and stick it in the washing machine with like colors.
It may take several washings to get it down to the right size, depending on the fiber composition. Mohair takes longer to shrink than wool, for example. The nubbly texture you see there is what happens when mohair worked in garter stitch goes through the wash several times.
To decorate, I needle-felted contrasting yarn into the Darwin fish on front, heathenish scarlet A on the back, and the border around the top. Embroidery would also work. To make buttonholes, just cut straight through the felted fabric and sew the same contrasting yarn around the holes.
I used roughly worsted-weight handspun mohair yarn and size 8 needles. You can use any weight of yarn you like and any size of needle that works with it, but if your computer is much larger than an Eee 901 PC, using anything much finer than worsted will take an awfully long time.
Also, this is irrelevant for instructions but it still makes me giggle: I needle-felted the big red A and Darwin fish into the laptop sleeve while watching the Matrix. Crafts, heathenism and geekery in one fell swoop. Where’s my Nerd Badge?