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:
- Adobe Photoshop - Feature Rich but Pricey. Takes a while to learn.
- GIMP - Feature Rich and Free, but also takes a while to learn.
- Paint.Net - Simple and Free
If you are uncomfortable learning Photoshop or GIMP, then Paint.net is a great alternative. You can follow their tutorials online or view youtube videos for creating transparent layers.