I have some pictures I'll post later, but here's the basic info on my bag.
I started with this pattern: http://tinyhappy.typepad.com/tiny_happy/2006/06/shoulder_bag_tu.html
Body: My finished bottom dimension is 14 inches where the pattern shows 17". Don't forget to add a seam allowance. I started out making an even bigger bag than shown, using 18" as the cut measurement, but found it more of a laptop/book bag size and not ideal for what I wanted. My next bag will actually be an inch or two smaller even. The cut height on the pattern is 10", my finished height is 11", again add a seam allowance to that. This shrinks some when you square up the bottom, my actual finished height is about 8.5".
Keep in mind when cutting patterned fabric that the "sides" of the pattern are the center of the finished bag and the center of the pattern is the ends of your bag. If you want to match up the pattern along that center seam, you'll need to lay it out carefully.
Strap length: I knew that I would be carrying the bag crosswise rather than on one shoulder so I made an oversize adjustable strap that I end up hardly ever adjusting. If I'm wearing something bulky I just put it under my coat. My strap is 27" from the top of the purse body to the center of the strap. If your fabric is cut fairly straight, you could get 25" of strap on an 11" cut body from a yard length of fabric and that's probably enough. My next version will have a patchwork strap, so I'm not worried about getting it all from one piece of fabric for the outside, only the lining. My strap finishes at 1 5/8" wide and I like that width which is much narrower than the pattern. Also, my curve from strap to body is more gradual, about a 6" radius for the circle (think dinner plate). I like that too and will do it again, I think it eases the transition to my narrower strap better.
Assembly: I wanted to have a pocket for everything, so my assembly involved quite a few more steps than the original bag. Follow the steps through sewing the front & back seams, but do not sew the bottom seam yet (bright blue on diagram). Also, I interfaced both of my fabrics which is not shown in the tutorial. Without an iron to fuse it, you'll probably need a sturdy fabric. Borrow an iron from someone at work or buy a cheap one, you only need it for a few minutes to fuse the interfacing to the flat pieces, but I use mine a ton when it comes to turning things prior to top stitching.
After stitching the centers, turn the lining so the right side of the fabric, the inside of the bag, is facing out and the centers line up (yellow dashed line). You will have something that looks roughly like the diagram below. Determine now how much you are going to square off the corner later. My bag is deeper than the pattern, I sewed my corner at 1.5 inches instead of 1". Don't sew the bottom or the corner, but you need to know this measurement now to place your pockets. Don't forget to allow for the bottom seam allowance, your RED vertical mark will be a seam allowance longer than your horizontal. I would suggest marking this with chalk or lightly with pencil so you can use it to place your pockets later.
Zippered pockets (purple line on diagram): this is the tutorial I used for the zippered pockets - http://u-handbag.typepad.com/uhandblog/2007/02/zippered_inner_.html If your machine does not have a zipper foot, you might be able to do this, or you can skip them. I like having them, they are huge inside my bag, I used 7 inch zippers and they go all the way down to the red horizontal line where the bag goes from side to bottom. If you are going to use a snap fastener later, make sure there is enough space between the purple line where you are placing your zipper and the top edge of the bag to allow for seam allowance, fastener and the width of your foot for top stitching. I would say about 2.5-3 inches. I put one on each side of the bag, centered on the seam. Make sure you fold the opposite side of the bag out of the way when you are stitching.
Open pockets: Cut two pieces of fabric for each row of pockets, I have them on both sides of the bag so 4 total. . Mine are 5 inches finished height, but your size will be determined by the size of your bag. Measure the area on your bag lining that matches up to the navy blue box on the diagram, add seam allowances, put the fabrics right sides together and sew around all 4 sides, leaving an opening for turning on one long side. Clip corners, turn right side out and top stitch along the long side with the opening only which will close it. Position each pocket with the top stitched edge towards what will be the top opening of the bag, with the bottom at or just above the horizontal red line, and pin in place. Fold back your zippered pocket and the far side of your bag and top stitch around the 3 remaining sides, attaching the pocket piece to the bag. This will give you one big pocket. I then sewed a few vertical lines on each side, dividing the pocket into sections for my wallet, notebook, pen, phone...etc. I found it helpful to have the items in front of me so I could get the sizes right. Keep checking to be sure that the zippered pocket and the far side of the bag don't sneak under what you are sewing. This will save you from time with the seam ripper later.
Fastener: at this point you can insert a magnetic fastener in the lining just above your zippered pockets. I used the center seams to make sure mine matched up. If you want a button & loop like the original pattern, this is the point where you go back to those instructions. Sew the bottom seam and the corners in the lining & outer fabrics and then follow the excellent directions for stitching up the rest of the seams, the strap and the opening in the lining.
Call me if you have any questions.