Don't forget the paper top and bottom before you iron, you will end up with a piece like this, after you have trimmed the edges.
It will go a bit wrinkly but that doesn't matter. Then I traced the letters I wanted from an embroidery magazine, onto the baking paper, and transferred them onto the plastic with a pencil and cut them out.Next cut eight pieces of plain plastic, I used a charity bag, slightly bigger than the panel size I wanted. Arrange your bits of decoration and cover the whole thing with a piece of clear plastic.
Save all your plastic bags, they come in usefull for cutting the pieces for your design, you can really go to town and personalise it. The more colours you have the better. You can also make up several of these panels and sew them together, it gives you a very strong piece which you can make into a bag.