Corners

Make sure to sew through all the layers to secure your corners. 

And start a new left to right row at the beginning of each side.

Finishing

On the back side of the hem make a small knot right at the point where your thread comes out of the fabric.

Pull the needle through the bottom edge of the hem, making sure it doesn't poke through to the front of the handkerchief.  Snip your thread.

Now find the tail you left at the beginning of the row and thread it. Pull your thread through the bottom of the hem and snip it, just like you did for the end of the row.

Once you've hemmed and finished all the sides the back should look neat and tidy, like this:

And the front should look like this: