Download and unzip the file to a place where you can easily find it. Open Affinity Designer, then open the Styles panel. If it is not visible, click View\Studio\Styles.
The Styles panel lets you apply pre-designed styles to your project such as bitmap fills, texture attributes and effects. To use a Style, select your object, then click on a style in the Styles panel and the program will apply that style to your object.
The styles on the following page(s) are bitmap fills. One thing to note is, if your object has a stroke around it, and you add a style, it will remove the stroke. You can re-apply the stroke in the Stroke panel.
Open the menu in the Styles panel (upper right corner) and select Import Styles Category. Navigate to where you saved the file and select it The styles will import into their own category.