I figure that you can make a pack by simply taking 2 or 4 sheets of fabric and lay them flat together (the fabric that you want as the inside liner on the outside.) And sewing a "U" shape into it. Cut around the edges of the "U" shape (and maybe lock stitch the edges for durability). Fold the tops of the "U" shape over and hem them to run a cord through and flip it inside out. Viola basic bag with draw strings. ( you can even cut holes in the front of the bag and hem them so the draw string will come out of the front. Also you can cut holes and have the string go in and out of the holes but this technique makes the bag liable to rip if you do not use grommets.)
Now there's many other things that you can do on how you install the straps, extra pockets (inside or out), internal support, padding, a rain flap for the top, etc. You may want to keep these things in mind when making the initial design.
For example if you are using four pieces of fabric and want a fancy french seamed rain flap that is already attached to the bag you'd have to make that first before you even started sewing the bag portion by making a smaller "U" shape above the first one is going to be out of two (one inner, one outer) piece of fabric then cut it around the edges and flip it inside out and the sew it to the other two pieces (with the liner on the outside so when you flip the whole bag everything's in the right place ) and put a hem underneath it for the draw string. So yea it can get pretty technical.
Like I said before takes some skills and it all depends on what you want and how much effort you want to put into it supplies etc. But I figured I'd throw something down for a basic design. I use the basic "U" design for a lot of my bags and if I need a small pouch to keep something in it works great and is easy to make.
Also this thread gave me a good idea on how to make a day pack and I thought it might be fun if I get the time and materials so that's why I was inspired to write this.
Hope this helps.
