Cut fabric 4-1/2" wide - one piece or pieced.  If pieced, seam can be anywhere from 1" below Closure (from my fist in the photo & towards "center") to fold "B".

Fuse interfacing to wrong side of fabric.

If fabric is directional, the end without closure marking is the outside flap, and the lower edge of this flap should be at the bottom of the design. (This edge is below the bottom of the photo at left & is the end without the closure marking, i.e. the snowmen will be right side up on the outside flap.)

If using Velcro or magnetic snaps as closures, stitch or make snips now to insert snap later. 

 The snap in the middle of the strip can be attached, but attaching snap at the end of the flap will make it difficult to stitch around end.  Do that after stitching and turning.

Looking at interfacing, press right sides together, fold A & fold B.  Fold them to meet at "center".

 

Fold on center, right sides together, matching flap ends (folds will not all be even - fold A extends beyond other 2, as it is the outside of the bag and needs to be slightly larger.)

Position strap ends (cord or stitched fabric) at box marking @ fold.

  

Stitch on solid line, leaving opening to turn through.  (Opening is to the left of my right thumb in the photo below.)

Reach inside opening & turn right side out.  

    

Both pockets should be under flap with closure on outside.  If not, turn pocket again.  If no closure is yet attached and you like the 2nd pocket on the back, leave it as is.  (See more photos below.)

If using magnetic snap, insert second part now, through snips cut earlier.

 

These photos show a purse with a contrasting fabric used inside (& for the strap) with Velcro closure.  The second photo shows both pockets under the flap. 

        

 The next 2 photos show the purse so that one pocket is under the flap and the other is on the back side. 

  

If you haven't attached the Velcro yet, you can decide if you want to leave it this way and attach the Velcro to match the flap.  And the 3rd photo shows both pockets under the flap, but the shorter pocket is inside the taller pocket.  If you have stitched your strap in the side seams, you might have a small problem, which can be solved with some creativity, if this is the look you like!

   

Finish opening, stitching by hand, or machine edge-stitching entire flap. (Actually, I used Liqui-Fuse.)

The flap can also be finished with a button & button-hole closure.  I like this, as I love buttons and they are such an easy embellishment!

The "Betty Boop" holder at left is actually holding a video iPod.  One pocket is under the flap, and the other pocket is on the back, holding the charging cord for the iPod.