This is not a beginner sewing project, but it's not extremely advanced either. You should have some experience using a sewing machine and knowledge of sewing terms like 'seam allowance' and 'presser foot' - or the willingness to look them up elsewhere!
Step 1: Tools & Materials
- 2 yd fabric (more or less, if you're making it REALLY big add another half yard)
- 3 yd 2" wide nylon strap 
- 1 24" zipper (or as long as you want the duffel to be)
- 2 8" zippers for the exterior pockets
- 2 12" zippers for the interior pockets
- 2 packets of double fold seam binding, one regular and one wide (I am actually using bias tape, but I'm using it AS seam binding and referring to it as such. Either will work)
- sheet of flexible plastic - I cut the bottom out of an old laundry basket
- 2 "swivel snap hooks" (see pic)
- 1.5 yd of nylon strap, of a width to fit your snap hooks 
- 2 D-rings or, my preference, triangle shapes
- 1 yd of nylon strap of a width to fit your triangles 
- a little bit of padding: quilt batting, old blanket, etc.
 Note that you may end up with nylon strap in three different sizes, or only one size; I used two sizes as my snap hooks took 2" strapping and my triangles took 1".
I used to have a nice picture of all the requisite pieces, but my camera ate it. Sorry!