The tutorial on Tattergarden has pictures of the process. I have tried doing the bags many different ways & the deciding factor is who it's for. I find I like a larger bag & a 13inch square works well. You have to factor in the turn down for the casing & seam allowances.
The bags I made for GDs could be reversed but as I made them like I would normally do a purse -construct the outer bag & lining (leave opening to turn and opening left for tape). Then with outer bag turned right side out & lining inside out, put outer bag into the lining & sew around top. Turn whole thing right side out.
Little things I like to do in making them:
-Use heavy interfacing on outer fabric & lining
-For larger bags make the way described above. For smaller bags -sew the way instructions on Tattergarden say
-handles I like best are fabric cut 4" x length desired. Iron in 1/2 to mark center, open out & fold each side to center mark & iron to mark. Iron in 1" x length of interfacing. Then refold along iron lines & sew closed (no turning!)
-sew handles on outside of bag -below the casing line. I sew them pointed to bottom of bag & then bring them up pointing to top & stitch to encase the raw edge.
Sorry to be so long winded but hope it helps