Step 1: Materials and Sizing
I was lucky enough to find a box that was just the right size for my macBook. You may have to hunt around to find one you like but as long as there are no creases in the middle (eg too small a box) you should be ok with any size.
Your box is perfect if you can just barely stand your computer on end when it's all constructed.
Step 2: Trimming, Part One
Cut your box open down one of the corners. Then trim it down to two sections and loose two opposing flaps.
There will be two measurements you will have to make now. The first corresponds to the depth or hight of your lappy. The second corresponds to the width. The easiest way to do these is to just roll it. First set it so the bottom lines up with the edge of the two flaps (like it is in the pictures) then set it up on end while keeping one edge on the cardboard like a hinge. If you are careful you will get the exact hight measurement you need. Mark with a pen. This measurement will be a crease. Keep rolling it so its lying upside down. Mark again. This measurement will be a cut.
You can crease however you like but I cut through the first layer down on the inside down a ripple of the corrugated middle layer.
see the photos. I remember something about 1000 words....