some time ago i made myself a protective laptop bag since i didn't want to spend money on something that seemed not too difficult to make. it turned out quite nice (i've included pictures at the last step), so a friend asked me if i could make her one, too.
so here is how.
this bag will fit perfectly tight around your laptop and with the thick, furry inside fabric and the sturdy outside fabric will provide nice protection from most things that will normally happen to a portable computer - if you don't start tossing it against a wall it will work fine.

my and my friend both have mac books, but i'm sure that this method will work well for any laptop that is a bit rounded at the edges and is not too thick when closed.

Step 1: Materials Needed

all the amounts of needed stuff are for a 13-inch macbook, for a different sized laptop you may need more or less!

- a piece of sturdy fabric for the outside, aprox. 60 x 50 cm for a 13 inch laptop, non-stretch cotton or similar
- a piece of thick, soft fabric for the inside, aprox. 50 x 35 cm, i used some sort of teddy fabric
- rest of a non-stretch fabric in matching color
- zipper, 60 cm long for a 13 inch laptop, longer or shorter if your laptop is bigger or smaller
- narrow double fold bias tape, around 3 m, around 5-6 mm wide when folded

- shears, pins, measuring tape, thread, ruler
- sewing machine

Step 2: Measure Your Laptop

measure you latop:

across in both directions, in my case (13 inch mac book) around 33 cm x 23 cm, millimeters don't matter, bring the measurement up to a round number
the thickness of the closed laptop, in my case 2 cm,
if the corners are rounded, measure more or less the radius of the rounding, im my case aprox. 1,5 cm
measure the thickness of the inner, thick fabric, in my case 0,5 cm

with this measurements, calculate the size of the "pattern" pieces:
add twice the thickness of the inner fabric (2 x 0,5 cm) to the first two measurments (33 and 23 cm), this means 34 x 24 cm, also add half a centimeter to the radius measure, so it will become 2 cm,
i've included a template of how the cover parts have to look.
you also need to know the circumference of these cover parts
either make a full size template yourself and measure once around,
or just calculate the lenght:
add the lenght of all the four sides minus the rounded parts, and then all the rounded parts, wich, added together are a full circle with the radius 2 cm (see above),
2 x 30 cm plus 2 x 20 cm plus (2 x pi x 2 cm) which is 112,5 cm.

Step 3: Cut Out the Pieces

from the outer fabric, cut out two parts according to the template from the previous step. and cut out a thin stripe of fabric that is a wide as the thickness of the laptop (2 cm) and as long as the circumference of the cover part minus the length of the zipper (see steps 1 and 2), in this case
112,5 cm - 60 cm = 52,5 cm.

because the inner white fabric is so thick, we can not cut the inside out in one part, because we need borders of thinner fabric around the thick one, otherwise it wil later be too thick to enclose with bias tape.
for the inner part, cut out
- 2 rectangles of the white fluffy fabric with a size of 20 cm x 30 cm,
- 4 stripes of black fabric 2 cm wide and 20 cm long
- 4 stripes of black fabric 2 cm wide and 34 cm long with rounded corners on both sides (see template)

leave seam allowance on all sides of all parts.

Step 4: Sewing, Part 1

sew both ends of the long thin fabric strip to the ends of the zipper, fold the seam allowance towards the fabric, then, from the right side, sew once along the seams, close to the seam.

sew one black 20 cm long strip to each of the 20 cm long side of the two white parts, then sew 2 of the 34 cm long parts to each of the obtained black and white rectangles, so that the rounded parts face outward.

on the two outside parts, mark wher the zipper will end: measure 30 cm in each direction from the middle of one of the 34 cm long side. mark with pins from the right side of the fabric.

Step 5: Sewing, Part 2

pin one inner part to one outer part, left side against left side. pin along the marked lines.

take one of the pinned together parts, and from the side of the outer fabric, pin the long stripe-zipper loop to the part.
place the beginning and the end of the zipper where you marked it in the previous step.
take care that the corners are nice and round!
sew the strip to the rectangular part.

trim the seam allowance all around to about 6 mm, then wrap (and sew) the seam allowance once around in bias tape. when you reach the end, fold the bias tape inside a bit to prevent fraying.

Step 6: Sewing, Part 3

now pin the second pinned to together part to the other side of the zipper strip, so that the parts that will later be outside face inwards.

again, sew the parts together, trim the seam allowance and wrap in bias tape.

Step 7: Reverse & Use

now all you have to do is reverse the bag so the ouside is actually outside and you're ready to use it.
if you took the measurements right, your laptop will fit perfectly tight inside the bag and will be well protectet.

i also have included some pictures of my first laptop bag, made with the same technique. it's made from blue artificial leather and has just thin cotton fabric on the inside - this is not as much protection as the one with the fluffy fabric inside, but it still works fine.
if you use thin inside fabric, you don't have to add the thickness of the inner fabric to the pattern and you can cut out the inside parts the same way as the outside parts out of just one piece.

have fun making your own laptop bag!