TY for this tutorial. I have made a lot of bags, but never one like this before. It turned out great. When my 9-year-old saw mine he wanted one in camo. You can see them both (and a matching toiletries bag) here:http://www.craftster.org/forum/index.php?topic=426071.0
Couple of notes. I used fusible fleece on the outter fabric and heavy weight fusible on the lining. I found it helpful on the second bag to clip the interfacing at the corners of the front/back panels. That way going around the corners with my sewing machine was less daunting. I also added a zipper pocket to the inside. The handles are not sewn into the seam but sewn all the way up the front and back panels. They are reinforced with a boxed-x that goes through both the lining and the main fabric. I'm pretty confident it will hold.
Someone asked how the lining is sewn to the main bag. This is what I did. Make the lining and the main bag. The zipper will be sewn to the main bag. Before you sew the pieces together for the lining prepare the zipper panels. I did this by folding under one edge about 1/2 inch on each panel and pressing. This is the edge where the zipper would set if it were there. Then complete the lining. There will be a gap where the zipper would be and that gap is where your folded edges are sort of facing each other. At the end, you will insert the lining into the main bag wrong sides together. Use the folded under edge and secure that to the underside of the zipper with lots of pins. From the outside of the bag, run another line of top stitching to the zipper panel. I just placed one row of stitches on top of the other topstitching, but you could place it below or above the top stitching - just make sure it grabs the turned under part of the lining. This will secure the lining to the bag. There will be a bit of a gap in the lining - that is okay and normal. You could hand sew this closed if you wanted to.
Alternately, you could sew the lining and the main fabric together for each pattern piece and then assemble it with the exposed seams on the inside of the bag. Sort of what you would do if you were quilting the bag. Then you'd have to cover the exposed seams with seam binding. But let me tell you, unless you use no interfacing - don't do it like this. That is how I made my first bag. Those corners are a bear to get around with so many layers. I have a heavy duty machine and sewing those corners was not pretty and my machine made some pretty horrible noises. Plus you have to go back over those same corners with that blasted seam binding. Yuck!
As for leaving a hole and flipping it inside out, like you might with other types of bags, that will not work with a bag like this. It has to do with the gusset/zipper combination.