It's really simple and quick to make - here's how...
1. Cut your exterior fabric, lining and interfacing to 46 x 28 cm (18 x 11 inches); a 1cm seam allowance is used. Iron the interfacing onto the back of your exterior fabric; you can also add some padding if you want your pocket to be more substantial (just bear in mind that this will take up some stashing space!)
2. Pin your two pieces of fabric right sides together and mark a 1cm seam allowance with a fabric vanishing marker all around the 4 edges of your fabric. I like to add the loop part of the hook and loop closure onto the lining at this stage to avoid the stitches showing on the outside of your pocket. Unpin 1 end of your fabric and sew an 8cm (3") length of the softer 'loop' halfway across and 2.5cm (1") up from the edge of your fabric.
3. Repin your fabric together and sew around your seam markings - remember to leave a gap of a few centimetres to turn your fabric through; backstitch at the start and finish to secure. Clip the corners so that they are easier to turn.
4. Turn your fabric through the hole (this is the best bit!), use a chopstick to point out the corners. Give your fabric a good press with the iron and then fold the end without the 'loop' fastener up towards the centre to create your pocket - I turned mine up to about 16cm which makes a nice deep pocket. Press the pocket crease.
5. Add the 'hook' part to the outside of your pocket - halfway across and about 2.5cm (1") down from the top pocket seam. The longer part is on the lining so that if your pouch is full, you can still close it.
6. Now for the topstitching - almost there! Unfold your fabric and topstitch along the top of your pocket (the edge above the smaller 'hook' fastener). Fold your pocket up again and pin. Topstitch along the three edges (not the folded bottom pocket edge) - the topstitching holds your pocket in place.
And there you have it - a handy little pocket to stash all the essentials. I'm hoping this means I can return to carrying my grown up handbags....everything Little Miss L needs in the lovely pocket and I can move it from bag to bag without forgetting anything. Goodbye, Peppa pig back-pack...hello Hermes Birkin ( well, I can dream).