Click on the Single Tile button. This is the fiddly bit. You need to paint changes in texture straight onto the map, which can require precision when dealing with slopes. There are helper tools available; including Flood Fill and the ability to apply a texture to just certain heights, but it’s still tricky - especially as this tool dumps textures down in squares. So click the Auto Edge In tool when you’re done, and then on the map. All your tiles will be blended together.