Pick up the stitches you first invisibly cast on. Here's where I put my opening to place the laptop inside, but I'm not sure it was the best choice. If you want to try that, knit 5 stitches, cast off all but the last 5, then knit 5. In the next row, knit 5, cast on just as many as you cast off earlier, then knit 5. Voila, a giant slit. If you don't put an opening here, figure out some other place to put it. Continue knitting up the rectangle that will cover the back of the laptop's display.