Do you want physical, tangible clouds? Or perhaps distant sky clouds?
For the former, you can use a plain textured with a cloud texture, ghosted, and oriented to face the camera. It looks more convincing if you use several smaller, randomly scattered, sized, and textured plains of this type, placed in a clump that represents 1 cloud.
For distant clouds, just use a very large ghosted, cloud-textured plain parallel to the ground. You can lock it's position relative to the camera position, or perhaps make it move a small fraction of the camera movement (to simulate distance). Try using more than one plain at different altitudes with different sizes, textures, and moving speeds. Also, experiment with scrolling the texture to simulate wind and/or player motion.
PS: See my sig picture: there are some clouds shown created by the first method I described.