Create a basic shape in Affinity Photo, it can be anything such as a heart design or arrow design. This can be done via the tools panel such as heart tool etc. You can also create a vector artwork using the pen tool and then manipulate the nodes using the node tool
Once you have created your artwork, the design can be duplicated by using the alt / opt key (PC or Mac) or by using the layer menu and duplicate command. The vector artwork can then be re-sized, make it perhaps a little smaller or just leave it the same as you can re-size the tile later.
Probably a good reason not to reduce the size is that the design is now going to be changed from a vector to a pixel layer via the layer menu and rasterize command as the selection into a pattern cannot be used with vectors. If you have any styles attached to the layer, de-select the check mark in the rasterize panel if displayed.
Select the pixel layer and go to the layer menu and then select the selection into pattern command and the design will be now turned into a pattern. The pixel layer is still there and can be deleted or hidden via the layer panel and clicking the entries 'dot' at the side of the name. The pattern layer can then be resized via the bounding box and going to the top middle of the box, it can also be rotated
You can now place the layer into the other layer by selecting it and dragging down into the original, only release once the entire layer entry is highlighted (not just the thumbnail). You can then select the original and resize and rotate that or select the pattern within the vector artwork and then resize and rotate that.
If you want to add some color, you can also go to the layer menu and add a new adjustment layer and drag that into the original design and place it above the tiles.