Imagine if your Photoshop CS4 Styles tab filled with a variety of styles; you will find it hard to work with the styles you want. To apply only the styles you want, follow this step (I am using Photoshop CS4, hope it will fit your program too).
First of all, turn off the Photoshop program you use. Then, copy and paste the styles you want to apply, for example, the one you buy in digital stores. Copy and paste the style with file extension .asl to Program Files (x86)> Adobe> Photoshop CS4> Presets> Styles.
Reopen the Photoshop program then click the down arrow in the upper right then open Preset Manager.
In the Preset Manager page, click the right arrow button, you will see that now you have the added styles. Click the name of the styles> done. The Styles is ready to use.