The Meaning of Life, the Universe, and Everything.
Lying on my Couch <3
Lying on my Couch =(^-^)=
Use something like Paint.NET or GIMP.
Make sure, the shape of the texture is a square, that means the height is the same as the width, and make sure that the size is a factor of 8.
Now select the top left tool in the tool window and select all the pixels of the texture you want to use, at the top is a bar of that tool of how that tool works, hover over everything till you find XOR and select the whole picture, that it marks anything you don't want to use and unmarks anything you want to use then you can either cut it out or just drag it out using the top right tool in the tool window, the later has it, that the unwanted parts won't be written into your clipboard.
I don't use paint.net, but check and see if there are different interpolation options for scaling images. Using nearest neighbor allows for a lower resolution image to be upsized without introducing new, usually semi-transparent pixels.