Same ways as before. Create a sphere and cut it in half. This time place it inside the sky sphere. You're going to need a cloud texture for this and photoshop.
You need to be familiar with alpha channels and the tools within photoshop to get this done. Basically you're going to apply a cloud texture all around the bottom half of this newly created sphere and animate the rotation to create the illusion of cloud movement. You can also achieve this with a poly sphere. You just have to layout your uv's properly, take a snapshot and apply the texture. It might be easier for you to do it this way. You can then also mess with the transparency to make the clouds seem transparent.
The main trick is to make the clouds seamless within photoshop using the clone tool and seamless tile tool. Takes a bit of practice. The most important thing is to make the alpha channel for the clouds, that way when you apply it everything else gets taken out except the clouds. Make sure you save your file as a tiff. THis format keeps the alpha info.