Here's the one that showed me the correct way to attach selvedge tape/bias tape/quilting whatsit that I use to finish off my edges so I don't have to worry about hemming stuff http://sewtospeak.blogspot.com/2010/04/how-to-attach-bias-tape-with-mitered.html
OK so that's all the credentials.
Now for the bag. I made a basic pattern for what I wanted by drawing it out on printer paper (you'll need 5 sheets) tape 2 sheets together for the body piece. here are the measurements, everything is rectangular, so just use a ruler or tape measure and straightedge.
Body and lining piece - 16 inches wide by 8.5 inches high
Pocket 1 - 10.75 inches wide by 4.75 inches high
Pocket 2 - 10.75 inches wide by 6 inches high
Zipper pocket 8.5 inches high by 5 inches wide (this piece has ended up too large for both the cases I've made with it but I trim it later and haven't bothered to measure how much it gets trimmed)
Here's what those pieces all look like, cut out. I drew the outline of my pocket placement on the "body" piece, that's what those lines are. You don't have to do this, there's very little precision involved.
You'll need a few supplies. I got all mine at walmart - about a half a yard each of your main fabric and your liner fabric, you can also buy interfacing if you so choose. I did for the purple case but I don't feel like it helped with rigidity, I was more concerned with how sheer the white fabric was and the pattern of the main fabric showing through. It does anyway, so you can skip the interfacing unless you're really inclined to use it. You'll also need a zipper (longer than 9 inches - mine's 15. you'll cut most of it off) matching thread and 3 yards of bias tape (the quilting variety.)
Pin and cut your fabric. You'll need one of each piece of both lining and main fabric so just lay them over each other and cut both at once to save time.
With wrong sides facing, sew the 2 pieces of Pocket 1 together 1/4" from the top.
Turn them right side out, pin and sew along the top again to make that pretty 1/4" seam.
do this for both long pockets (Pocket 1 and Pocket 2) then set them aside.
With right sides facing out, sew the 2 pieces of your zippered pocket together along all 4 sides.
Pin zipper to the zipper pocket piece(right side of fabric facing up, zipper laying face down with top edge lined up as shown) and sew. I don't have a zipper foot, so my seam isn't very close to the metal part of the zipper, but make sure they're attached all the way across.
Turn the zipper up so that the right side of the fabric is up and the right side of the zipper is up as well (shown) pin and sew to make that seam lay flat. Here's a back view.
Pin and sew a piece of bias tape to the other side of your zipper. Don't bother unfolding it, just pinch the zipper between the 2 folds of bias tape and sew straight across.
With right sides facing out, sew the lining piece and body piece together along all 4 sides.
Liner side up, measure 10 inches from left-hand side of body piece, line up BOTH long pockets (they should also be liner side up) pin and sew in place.
Flip both pockets around so that the main fabric faces out, edges should line up with the left hand side of the body piece now. If they don't, make sure the pockets lay flat and the edges can hang over and just trim the excess off later, you want your pockets flat or your knitting needles will fall out. Pin the right-hand seam (the one you just made) and sew across again to make the edge look nice.
Sew across the left-hand side and bottom of the long pockets as well.
With your ruler, measure the length of the pockets from seam to seam and divide into thirds. Draw light lines with a pencil and sew along those lines from the bottom up to create your smaller pockets. You should now have 6.
Line up and pin the zippered pocket in place. The bias tape should be touching the longer pockets you've already attached. Sew along the bias tape 1/4" from the edge.
This is where that overly-long part of the pocket comes in. Pin the other 3 sides of the pocket to the body piece and sew them down. You can see in this picture from the back side of the work that the pocket itself overlaps by about an inch, make sure you're sewing it to the body all the way around. You will also sew over both sides of the zipper in this step - make SURE your zipper pull is inside your work when you do this or you won't be able to use it.
Trim the excess zipper as well as the overhang from the zippered pocket.
Unfold and pin the bias tape (start in the middle of the long side of your work so you don't get nasty corners, and make sure you tuck the raw end of the very beginning of your bias tape under.) pin one side, sew to 1/4" from the corner.
At the corner, just force the tape around and start pinning it to the next side, sew, and repeat for the remaining sides being sure to backstitch well at the corners so it doesn't pull up. Overlap an inch or so at the end to hide the raw edge.
When you've got the bias tape attached all the way around, begin folding your bias tape back together and pinning it in place on the other side. Make sure the crease sits flat on the side you already attached before wrapping it around. Pin it completely folded over, hiding raw edges and seams all the way around. Tuck corners neatly (ish.)
Sew around, remove pins and snip stray threads. You're done! Now you can load it up with all your junk.
I used a piece of bias tape to tie it closed for display purposes, you might want to use a piece of ribbon for yours.