This is your standard A4 Sized paper. You can use any size paper, follow the next few steps to get a square!
Take the top right corner and fold it to the bottom left size, make sure the left corner where the paper meets are as sharp as possible to make the square as close as possible!
Using either a ruler or scissors, cut along the line below the triangle you made!
After you cut, you'll see that if you cut with a ruler, there'll be paper fibers where you cut it but it doesn't really affect the origami in the end. If you're really bothered you can use a scissors or a penknife.