Zip it up (Fly Zipper Insertion)
Added Jun 27, 2012
You can use a fly template (taken from any other trousers) if you are not comfortable drawing it yourself. I draw it by hand, using a ruler and rounding it at the bottom. Place a row of nice even stitching over the line, making sure it doesn't catch the left flap underneath ( it must be attatched only at the bottom.)
Make sure you backstitch at the centre seam at the bottom to strengthen the fly.