Now it's your turn. Open up a texture in GIMP. Work out roughly how large this texture would be in real life. Take 20 pixels per meter and resize the image to the correct number of pixels. Use the quick tile filter as you've learned, use some color or contrast adjustments if you want, and create a selection mask. Easy as that!
You can make multiple selections with the Fuzzy Select Tool by holding the Ctrl (or Cmd on Mac) key.
You can use the final image shown earlier in the chapter to check what goes where. Remember, this doesn't have to be perfect at this stage. It's about learning. In addition, any of your textures can easily be replaced later if you wish to by pasting into the layer ...