1- Open Affinity Designer.
2- Select the shape tool and create a shape. Fill with colour using the colour picker tool.
3- Select the gradient tool.
4- Select the shape. In the top toolbar select the type dropdown box and select bitmap.
5- A new box will appear, find the texture and select it. then click open.
6- You will see the texture now filling your shape.
7- With the gradient handles you can adjust the size of the texture.
8- To change the texture re select bitmap in the type drop down box and then select a new texture.
9- You can even add images as a fill. This is a great way to create you own framed pics.