Best advice i could give, after having made 5 satchels, is just go for it!
Get your inspiration from others work, but forget patterns from other people and design your own. They are really not that difficult once you break down what you need....
....ie. A front panel, a back panel that folds over to create the front flap. A side piece that is one long continuous side/bottom/side. (You may want two of these if you make a bag with two compartments. Various straps for the buckles, handle etc. If you want to add outside pockets, they tend to be mini versions of the main bag.
Get some large thin cardboard sheets, and start drawing at actual scale. Cut the shapes out and use a stapler to fasten them along your proposed stitch lines....you will soon see if things are out of whack. Once you have a 3d cardboard bag that is a good representation of your bag, unpick the staples and you have a ready made cutting pattern.
All my bags were made this way, and if you look at the images in my gallery, you'll probably see enough detail to get an idea how i made everything.
Theres rarely such a thing as a completely new design, but you will feel much more satisfied if you are carrying a unique piece that you can honestly say is all your own work. Plus you will learn so much more than just following someone elses working out.
That a good looking bag - good luck with your own !!
and i agree with Cheryl - you need to prioritise the order in which things are assembled otherwise you will find yourself in a world of pain trying to attach things that should have been added earlier on!
adam