First, flip over the opened end of the tie/ strap and then place the tool that you are using, (in this case I am using a crochet hook) in between the flipped fabric and the non-flipped fabric.
While gently pushing up on the tool, pull down on the flipped fabric.
When you are finished, if you are using a tool with a pointed end you can lightly poke the corners to make them nice and square.
If neither side of the strap/ tie is sewn shut than you can simply attach a safety pin to one side and flip it inside the strap.
Then push the safety pin towards the opposite end of the strap. (This is similar to putting elastic through a waistband.)
When you get to the end take out the safety pin and you are done. Voila!