Using discontinued Wallpaper books available at Paint and Paper stores, I've selected several pages and made gift pages from the individual pages. Here's a picture of a couple of books I recently picked up from one of our local Paint & Paper stores, plus a picture of several pages I cut out of the books.
The width of the page varies depending on the wall paper book. The sample I've shown below was 15 1/2 inches across. In order for the finished bag to be squared, you need to decide what size you would like your gift bag to be. Keeping in mind the wider you want the bag to be the small the front and back will be. In the example below, I've allowed 2 inches at the left side. In order to have the same width on the opposite side I have to allow 2". For the flap at the right hand side I've used 1 1/2 inches. As you can see, I've used up 5 1/2 inches on the width of my bag so far (2 + 2 + 1 1/2) Subtracting that 5 1/2 inches from the overall width of 15 1/2 inches leaves 10 inches. Divide that amount by 2 and I have a bag that will be 5 inches across. You can work with any width of paper, if you start by marking the inches for the sides first, that will leave you an amount to divide by 2 which equals your front and back on any bag. 
The turn up on the bottom of the bag has to be the same number of inches on you are using of the left hand flap - in this case 2 inches. Again this is so the bag will be squared when it's finished. From the picture below, you can see that I've also allowed 3 inches for the fold down flap at the top, however, you could make it 1 inch if you wanted a taller bag. Using a ruler I fold along my penciled lines and then run the handles of my scissors of the outside of the fold for a sharp crease. I also cut the bottom flap as shown on a slight "vee" shape to make it easier for gluing. 
Fold the top flap down and then turn the paper over so the right side is facing. Put white all purpose glue on the 1 1/2 inch strip, from top to bottom as shown 
The 1 1/2 inch flap will be glued on the outside and then slipped under the flap showing of the left hand side of this picture
Slip the glued flap under the flap on the 2 inch side. The glued edge will be on the inside of the 2 inch flap. Make sure the join is even from top to bottom and then use 3 or 4 paper clips or clothes peg to hold in place under the glue dries. After the glue has dried, removed the paper clips and apply the white glue to the 2 side flaps and the outside (good side) of the back panel. Fold the two flaps in and carefully fold the long flap with the glue on top of the flaps, then fold the last flap onto the glued long flap. You can put your hand inside to press down to make sure everything is stuck together. Let dry completely.
Bottom of the gift bag, with all the flaps glued and folded in.
You can now complete your by adding handles and a gift tag. You can use a few strands of raffia twisted together, macrame cord, or thinner cord purchased on spools from the dollar store. You can also cut strips of wallpaper about 1 inch side by 12 inches long. Fold the wall paper in half lengthwise, run glue down one edge, fold and use paper clips to hold in place until the glue sets.Add glue to the end of the strip and place inside the bag about 1 inch from the edge and use a paper clip to hold in place until glue sets. If you use raffia or cord, punch holes in the bag at each end, thread the cord through (right side to inside) and knot. Using a strip of wall paper about 2 inches by 4 inches, glue a plain piece of paper inside and trim. Once dry fold so you have a 2 by 2 inch tag. Thread crochet cotton or any heavier type cotton thread through the top corner of the tag and tie it on your gift bag. If you decide to make several at the same time, once they are completely dry, they can be folder the same way the store bought bags are folder which makes it easy for storage.
If the paint and wallpaper store in your area doesn't currently have any discontinued wallpaper books, ask them if you can put your name on a list so they will call you when they become available. Another place to get wallpaper is from thrift stores or yard sales. People often have left over rolls they put out for sale. If you manage to get a roll, you can make much bigger bags. I would suggest you practise making a bag on a sheet of wallpaper you don't particularly like so you can play around with it perfect your technique. Have fun.
Click HERE for a link I found years ago for making crafts with wallpaper. Several of the attached links in the original thread no longer work, but it may still help you come up with ideas of your own.
