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Copy paper is extremely common, cheap, and easy to obtain. If you're fine with using paper that isn't blank, you can often get large amounts of used paper for free. The only thing keeping printer paper from being "origami paper" is the fact that it is rectangular, not square. You will need to trim some away for proper origami paper. Properly folding the printer paper will allow you to cut it into a perfect square without using a ruler. Take the top right corner and fold it down until it touches the left edge of your paper. The entire top edge of your paper should now be even with the left side. Make a crisp crease along the fold. Your paper should now look like a sailboat with a folded right triangle "sail" sitting on top of a single-layer rectangle. Take the point in the top left corner and fold it down so it is equal with both the left side and the base of the triangle. Your paper should now look similar to a house. The top will now be a right triangle with a centered point and the bottom section a rectangle. Take the rectangle at the bottom and fold it behind the triangle. Make a sharp crease along the edge. Now you can unfold the triangle. This will remove the excess paper. Unfold the paper completely. Use a pair of scissors to cut off the bottom rectangular flap. Use the crease to guide you, and try to cut as straight of a line as possible. Now you should have a square piece of paper that you can use for your origami practice. You may want to use a hard, flat object to smooth the paper so that it's easier to work with when you are folding your origami. Try putting it inside a thick textbook for a day or two.
Gather normal copy or printer paper. Make your first fold. Make your second fold. Fold the bottom flap. Cut along the bottom crease with a pair of scissors. Unfold your paper completely.