I used a LOT of invisible cast-ons in this piece so it all fits together and appears (mostly) seamless. Once you've knitted the bottom rectangle (with invisible cast on, of course), you can choose how to proceed. There really is no BEST way to do this, so here's what I did:

Start knitting a few rows up to what will be the front edge of the laptop. Now it's time to add the sleeves (remember, your hands come off your laptop at a steep angle, not straight up). Determine how wide your sleeves will be and count out enough stitches to have them centered along the front edge of the laptop. Knit to this marker, then place the sleeve's number of stitches on a holder, place the divider stitches on another holder, then do the same yet again for the next sleeve's stitches, then knit to end. I knitted about ten rows up on the sides and the divider section before invisibly casting on the same number of stitches that are on the sleeve holders. By now you'll have two squarish holes in an otherwise rectangular piece. Pick up the holder stitches, pick up stitches along the side of the hole, pick up the invisibly cast on stitches, then pick up more stitches along the other side of the hole, using double pointed needles. Knit in K1, P1 rib for as long as you'd like your sleeve to be. Repeat for the other sleeve.