I partially based it on my everyday satchel, which I made back in January using this tutorial. It's a great functional bag with lots of pockets (and, crucially, is much lighter to carry than my old leather satchel). The laptop bag differs in that it has roomier front pockets with velcro closures, but no zip, no inner pocket, and no back pocket.
A peek under the flaps...
|Everyday bag: quick-access front pockets for bus card etc.|
|Laptop bag: velcroed front pockets for cords; no zip.|
To get the size right, I made a paper template by drawing around the laptop and then adding an inch to each side to allow for seams and a bit of extra roominess. I made the lining first because it seemed like a more straightforward place to start, and then made the strap and front pockets. The strap is made of corduroy, which means it has less of a tendency to slide off my sloping shoulders. I first tried the corduroy trick on my everyday satchel, and it's made a big difference! I love finding solutions to little niggling things like that.
I attached the front pockets to the main front piece of the bag, and then basted some cotton fleece to the wrong side of the main front and back pieces, and sides+bottom strip. This gives the bag some sturdiness, and will protect the laptop against minor bumps and knocks.
The finishing-off part of the process (attaching the lining) was a bit of a brain-bender, but referring back to the tutorial helped. I certainly don't find sewing bags easy, but I do love the end result of a custom-made bag that's just what you need. :)