Chose the font tool and select a nice chunky font without any thin flourishes as these might be too fine to cut well. For this one I used rounded arial and chose 300 as the size.
Go to layers and merge visible.
Chose the rectangular selection tool and use it to draw a large rectangle over the image making sure it covers all of the image. Click anywhere inside and the lil dashed lines will be all the way around the edges of the image. Chose the paint brush tool and colour it in. For speed I use as large a brush as possible.
Click selections, select none then layers, duplicate layer. Now click image/ mirror.
Your image should look something like the one below.
If you look closely you'll see that the 2 layers are larger than the image, go to image/canvas size and enlarge the canvas width, I changed to 24 * 24. The size doesnt matter as long as it's big enough to fit both layers side by side and a bit of spare space.
Zoom in until you have a really good view of the top of the image. Use the move tool to get both layers joined up. It's important to get them lined up accurately otherwise when you fold the back wont be the same as the front.