before you render your clouds (don’t forget to disable blending before you render normal geometry again).
Also, with this type of blending, you’ll have to ensure that you render back to front, and it’s generally a good idea to enable the depth test but disable depth writes when you render transparent textures.
You’ll have to make up your own mind on whether or not you want to use an alphachannel for blending.
If the whole bottom texture is cloudy but with lighter and darker areas, you’ll probably get a good effect with just a fixed alpha for the whole quad. If you’re going for loose clouds, use an alpha channel in the texture.