Well here it is! A tutorial for the exploding handbag.
First cut your A4 piece of card to measure 21cm x28cm. Score at 7cm intervals on both long and short sides. I've also made a score line 1/2cm away from the final score line in the centre section at one end. This is to accommodate the bulk of the card when it is finally folded in.
Remove the two corner pieces as indicated by the blue diagonal lines above. Make sure to remove them from the end where you made that little extra score line in the previous step.
Make a tick mark on either side, 10.5 cm along the side of the base. The flap was NOT included when measuring this.