I could try to think back through what I did. I don't have a program to draft up plans though. Feel free to ask questions.
The first thing to do is learn how to use a thread injector (sewing machine). The best place to start for me was making hammocks. You can make the ugliest hems and it doesn't matter, as long as they are strong. Next, I started making tarps, which taught me how to make box stitch (that might not be the exact phrase - it's a box with an X in the middle). It's pretty simple stuff. You just need to buy heavy duty yarn. Check out backwoodsdaydreamer.com for supplies.
For this bag, I took my 6 discs that I wanted to carry and wrapped them in fabric. I pinned the fabric together with the two loose ends of fabric pointing the same direction (think bag, not burrito). I took a pencil and marked where I needed my first hem, then sewed it up. Next, I added a second hem behind the first to add some more strength (though it might not have mattered). Then, I stitched the bottom closed with a single row, so now it was literally a bag with one open side.
On the open side, I did a 1" roll hem, then rolled it over 2 more inches and did another hem - 1 line near the top, and one close to the 2" mark. After that, I put the discs in the bag and found out what to cut from the corners - first I measured, then marked the corners where I wanted a hem, sewed a hem, then cut the excess off.
After that, it was time for creativity to kick in to figure out how to attach all of the reinforcements and straps.
Oh, and make sure you work with the bag inside out, so when you are done, all the hems are inside the bag and it looks cleaner.