Start with your paper so that it is taller than it is wide and then tri-fold it.
Open your paper back up and then fold two of the corners so that the meet the crease on the right
Take the bottom of you paper and fold it so that it meets the folded down corner on top. Repeat with the top edge of the paper.
You should have a nice square folded in the middle by this point
Take the bottom corner and fold it so it meets with the top crease. Repeat this with the other 3 corners.
You should now have X's surrounding your square
Fold the right side back over.
At this point I would usually add a design to the rectangle on top. It's easier to do it now than when the bag is done.
This part gets tricky and I hope you can follow the pictures. Use the X in the middle of the right tri-fold as your guide
You are going to repeat this process with the left side of the paper. Yeah, it's probably even trickier now. You are now using the X in the left tri-fold as a guide
Take this bottome corner of the X and fold it so that it meets with the top left corner of your center square
Pinch the top of your bag closed
The original instructions I found for this bag continue to do some more folds and the bag locks itself closed. I just punch a couple of holes in the top.
Then I run some ribbon or yarn through the holes.
Then I fill up the bag and tie a bow.