You have probably noticed that creating a Template Background is very simple, while creating a Template Overlay requires more work. 

The simplest way of getting an Overlay is to purchase a stock Overlay from one of the many template design websites. 

However if you cannot find what you want, you may also create one from scratch. There are 3 main Softwares that can do this:

  1. Adobe Photoshop - Feature Rich but Pricey. Takes a while to learn.
  2. GIMP - Feature Rich and Free, but also takes a while to learn.
  3. Paint.Net - Simple and Free

If you are uncomfortable learning Photoshop or GIMP, then is a great alternative. You can follow their tutorials online or view youtube videos for creating transparent layers.