Quixel is a fun and intuitive way to create interesting mixes of any material you can imagine. Once it is all set up you can export textures straight in to Unreal at the click of a button. 

1. With Quixel Mixer open you can use the local library or online tabs to add a material layer.
2. You can experiment with the blend tab to control the layering of materials and push one up through the other in a realistic way.
3. Add a paint mask to your layer with the button in the bottom of the layers panel. (You may need to press the restore down button in the window (next to minimize) and then restore up again.
4. Draw with the paint mask to mask out parts of the layer.
5. You can change the albedo of a texture by editing the colour in the albedo section directly.